Since its inception, OpenJDK 6 has been using HotSpot 10 with a series of patches to fix bugs, etc. However, the 6 update train has been upgraded to HotSpot 11 and OpenJDK 6 should do likewise.
The bugs fixed in HS11 include:

6337834: JVM should remove unneeded zeroing during object allocation
6538910: CMS: excessively long abortable preclean cycles
6579537: CORBA:api/javaidl/StubTests & product/rmiiiop/marshalTests tests are failed with core dump
6571496: Bigapps Failure- the app is hanging with the specific VM
6616627: 64-bit j2se build problem: ArrayIndexOutOfBoundsException: -2147483521
6614036: REGRESSION: Java server x86 VM intermittently crash with SIGSEGV (0xb)
6395208: Elide autoboxing for calls to HashMap.get(int) and HashMap.get(long)
6642634: Test nsk/regression/b6186200 crashed with SIGSEGV
6497639: Profiling Swing application caused JVM crash
6599425: OopMapCache::lookup() can cause later crash or assert() failure
6631248: Memory problem when doing invalid type cast
6623167: C2 crashed in StoreCMNode::Value
6610906: inexplicable IncompatibleClassChangeError
6663621: JVM crashes while trying to execute api/java_security/Signature/SignatureTests.html#initSign tests.
6659207: access violation in CompilerThread0
6661247: Internal bug in 32-bit HotSpot optimizer while bit manipulations
6676016: ParallelOldGC leaks memory
6614100: EXCEPTION_ACCESS_VIOLATION while running Eclipse with 1.6.0_05-ea
6662086: 6u4+, 7b11+: CMS never clears referents when -XX:+ParallelRefProcEnabled
6681796: hotspot build failure on gcc 4.2.x (ubuntu 8.04) w/ openjdk6
6620329: SA: Jstack prints doubled native methods on Solaris/Sparc
6707044: uncommon_trap of ifnull bytecode leaves garbage on expression stack
6722112: CMS: Incorrect encoding of overflown object arrays during concurrent precleaning
6739357: CMS: Switch off CMSPrecleanRefLists1 until 6722113 can be fixed
6741642: bad enum definition in ciTypeFlow.hpp
6722116: CMS: Incorrect overflow handling when using parallel concurrent marking
6732194: Data corruption dependent on -server/-client/-Xbatch
6758234: if (k cond (a ? : b: c)) returns reversed answer if k is constant and b and c are longs
6757316: load_constant() produces a wrong long constant, with high a low words swapped
6758445: loop heads that are exception entry points can crash during count_edges/mark_loops
6767587: missing call to make_not_entrant after deoptimizing for patching volatiles
6695777: Queens.class should be built from source, not put in source repo
6710791: Remove files or build them from source: maf-1_0.jar, jlfgr-1_0.jar
5073453: problems with ACL on the hsperfdata_<user> directory on windows
6272174: -Djava.compiler=NONE and -Xdebug anachronisms unanachronistically disable full-speed debugging
6397628: PIT : Native libraries should be built with correct g++ flags
6405858: TEST_BUG: regression test runtime/4482614/ExecTest.java goes into infinite loop
6460014: Tidy up task queues
6464263: Bypass ConvI2L operation for positive array's index on amd64
6471009: Significantly higher CPU usage in jvm1.6 build 97/98 vs jvm1.5.0_06-b05 on DOTS ATCJ2 test
6487931: JVM reports EXCEPTION_FLT_STACK_CHECK when calling a Java method through JNI
6506133: Hotspot JVM Out of Memory Error in JDK 1.6RC1 but not in JDK 1.5.0_09
6506252: Do not mark cards for objects known to be in the young gen
6506313: TLS shouldn't be rematializable on win32
6510732: Compute consistent block frequencies
6511756: forte_is_valid_method() should call CollectedHeap::is_in_reserved() rather than is_in()
6518571: Assertion failure with fastdebug VM on windows-amd64 when -agentlib:jdwp is used
6519515: Loop-opts incorrectly removed a safepoint poll from a loop with an early exit
6529811: LogCompilation shows infinite repetition of uncommon_trap action='none'
6530811: regression b08: SEGV in FastScanClosure::do_oop
6531227: Allocate-prefetch only once to a given cache line
6538429: Make HotSpot pscan clean
6546278: Synchronization problem in the pseudo memory barrier code
6546452: RFE: Provide a way to diagnose the java application if safepoint take too much time
6550155: Reduce the size of VM
6550813: JVM crash with EXCEPTION_FLT exception with product Alcohol 120
6551457: JVMTI: reference_info in HeapReferenceCallback is not NULL for "otherwise" reference_kind
6552746: UseAdaptiveGCBoundary for expanding does not align correctly.
6558100: CMS crash following parallel work queue overflow
6558600: need support for allocating objects at deoptimization
6561023: TEST_BUG: some tests fail when run for particular configuration
6561055: closed/compiler/4998314/Test.java causes core dump
6561906: Assert in DisoveredListIterator::load_ptrs() too strong in certain execution contexts
6562569: REGRESSION: can not see BLOCKED state change after patching 5104215 diff.
6567018: Hotspot build fails with gcc4
6569768: CMS: System.gc() may hang with -XX:+ExplicitGCInvokesConcurrent upon concurrent mode failure
6570219: REG: VM crash when printing an image in different orientations, SolSparc
6570268: JDB: step002 fails assert(0 <= i && i < length(),"index out of bounds")
6571184: TEST_BUG: the timeouts for several tests need to be increased
6571248: Open source developers would like an example jvm using c++ interpreter
6572484: Javac crashes hotspot VM with -server -Xcomp options on windows, 64 bit Solaris SPARC/AMD
6574694: Fix JVM TI versioning for HotSpot Express
6575068: Hotspot Windows build fails in VS2005 due to getcwd/_getcwd POSIX name change
6575679: jbb regression 2-3% in b15 on x86
6575876: java -server -version should not indicate tiered VM
6576627: Error: assert(which == imm64_operand && is_64bit,"")
6577180: JVM crashes when multiple LineNumberTable attributes exist and the last one is empty
6578245: Internal Error (relocInfo_amd64.cpp:67)
6578344: assembler_i486.hpp won't compile with gcc4.1.2
6579347: Solaris/SPARC Server VM: endless loop in checkcast_arraycopy stub
6579506: assert(!o->is_null_object(),"null object not yet handled here.") on CTW runs
6581118: For GC ergonomics on 64b server-class machines, remove the 1G maximum on default heap size.
6582890: assert(cost >= 0,"negative spill cost")
6584342: assert(hd->head()->is_Loop(),"must begin with loop head node")
6584839: CheckUnhandledOops flag SEGV's some nsk tests
6585490: inline_unsafe_prefetch() throws assert(saved_sp == _sp, "must have correct argument count")
6586272: uncommon basic blocks not moved to end
6587066: ~Thread() assert(last_handle_mark() == 0,"check we have reached the end")
6587871: HotSpot sa.jar build target should build the full sa.jar
6587896: TEST_BUG: closed/runtime/4784641/CheckedIsSameObjectTest.java fails
6588045: Pack: java.io.IOException: pop token out of range
6588497: jvm crashes on windows amd64 Vista exit status: -1073741819
6588598: incorrect assert in fast arraycopy code
6588638: improve support for large pages
6589176: SuperWord for 64 bit
6589186: 20070719212032.jrose.6337834 putback produces amd64 jvmg error: Bad immediate dominator info
6589804: assert(allow_null_referent ? _referent->is_oop_or_null() : _referent->is_oop(),"bad referent")
6589868: transition to Mercurial: need to eliminate dependencies on SCCS keywords
6589875: guarantee(klass_hash_ok(k, id),Bug in native code: jfieldID class must match object)
6590301: HotSpot VM should have own version separate from JDK version
6591159: Refine assertions and better fix for 6530811
6592476: redudant spill moves no longer being eliminated
6592707: REGRESSION: Java 64-Bit Server VM sparcv9 crash with unhandled implicit exception in compiled code
6594051: 30% regression in scimark-small on x86 64-bits VM
6595052: JPRT isn't running solaris tests
6597143: ThreadLists fails: inconsistent results
6597720: hotspot/src/share/vm/gc_implementation/parNew/vmStructs_parNew.hpp contains wrong legal notice
6597724: hotspot/src/share/vm/utilities/copy.cpp contains wrong legal notice
6597884: "java -XX:+PrintSafepointStatistics -version" crashes
6598210: Assertion error: loop exited more often than entered
6600952: SA should print warning when SA and VM not release versions are not matching
6601377: hotspot/src/share/vm/prims/jvmtiLib.xsl generates files with incorrect legal notices
6601384: hotspot/src/share/vm/adlc/archDesc.cpp generates files with incorrect legal notices
6602306: PS, ParNew: reference processor's _is_alive_non_header field should be left NULL
6602875: scimark.small 10% regression with 64-bits VM
6603492: multianewarray should generate same code as anewarray for 1-d arrays
6604006: globals.hpp should support non accumulating ccstr arguments
6604443: Bigapps fastdebug: Attempt to use MPSS failed
6604499: CHA code is no longer used and should be removed
6605173: par compaction should go on a diet
6605177: parallel gc should not require a large page for the perm gen
6607264: Java stack trace on error broken for interpreter frames.
6607945: performance regression after fix for 6594051
6608692: HotSpot version wrong when JPRT is used to build on Linux and Solaris
6611670: -XX:+VerifyDUIterators assert(_del_tick == node->_del_tick,"no unexpected deletions allowed")
6612113: can't build optimized C2 VM
6629887: 64-bit windows should not restrict default heap size to 1400m
6635560: segv in reference processor on t1000
6670684: SA command universe did not print out CMS space information
6724218: Incorrect code is generated for org.apache.commons.collections.list.AbstractLinkedList::removeNode