I've been seeing this assertion failure periodically in nightly.
Here are the entries from my nightly analysis report:

New nsk.quick_jdi failures (from 2007.12.05)
    nsk/jdi/BScenarios/hotswap/tc09x001
        This test failed the following assertion:

            Internal Error (src/share/vm/code/relocInfo.hpp:1089)
            Error: assert(target != __null,"must not be null")

        on Linux IA32 Client VM (machine colfax003). There is another
        instance of this assertion failing in the filter_rt005 and
        tc01x002 entry below.

http://sqeweb.sfbay/nfs/tools/gtee/results/JDK7/NIGHTLY/VM/2007-12-05/Serv_Baseline/vm/linux-i586/client/comp/vm-linux-i586_client_comp_nsk.quick-jdi.testlist2007-12-05-22-37-09/ResultDir/tc09x001/


New nsk.quick_jdi failures (from 2007.11.29)
    nsk/jdi/ClassLoaderReference/visibleClasses/visibleclasses002
        This test failed the following assertion:

            Internal Error (src/share/vm/code/relocInfo.hpp:1089)
            Error: assert(target != __null,"must not be null")

        on Linux IA32 Client VM (machine colfax003). There is another
        instance of this assertion failing in the filter_rt005 and
        tc01x002 entry below.

http://sqeweb.sfbay/nfs/tools/gtee/results/JDK7/NIGHTLY/VM/2007-11-29/Serv_Baseline/vm/linux-i586/client/comp/vm-linux-i586_client_comp_nsk.quick-jdi.testlist2007-11-29-23-03-18/ResultDir/visibleclasses002/


New nsk.quick_jdi failures (from 2007.11.03)
    nsk/jdi/StackFrame/getValues/getvalues003
        This test failed the following assertion:

            Internal Error (src/share/vm/code/relocInfo.hpp:1089)
            Error: assert(target != __null,"must not be null")

        on Linux IA32 Client VM (machine colfax003). There is another
        instance of this assertion failing in the filter_rt005 and
        tc01x002 entry below.

Update: No nightly link avaible for the 11.03 failure; too old.


New nsk.quick_jdi failures (from 2007.10.27)
    nsk/jdi/WatchpointRequest/addClassFilter_rt/filter_rt005
    nsk/jdi/BScenarios/hotswap/tc01x002
        These tests failed the following assertion:

            Internal Error (src/share/vm/code/relocInfo.hpp:1089)
            Error: assert(target != __null,"must not be null")

        on Linux IA32 Server VM (machine colfax003). These tests passed
        in the 2007.10.26 nightly on machine dip running the same
        config.

Update: No nightly link avaible for the 10.27 failure; too old.


So far this failure has only been seen with Linux Client or Server VMs.
I'm putting the bug in hotspot/compiler1 since it has been seen most
often there.
I found the hs_err logs for the earlier failures as well and they are exactly the same though the code had changed slightly.
Another failure from nightly:

New nsk.quick_jdi failures (from 2008.01.14)
* nsk/jdi/VirtualMachine/canGetOwnedMonitorInfo/cangetinfo001
        This test failed the following assert:

            Internal Error (src/share/vm/code/relocInfo.hpp:1089)
            assert(target != __null,"must not be null")

        on Linux IA32 Client VM (machine colfax003). This is an
        occurrence of the following bug:

            6644928 4/4 Internal Error (src/share/vm/code/relocInfo.hpp:1089)

        I will copy this entry to 6644928.

http://sqeweb.sfbay/nfs/tools/gtee/results/JDK7/NIGHTLY/VM/2008-01-14/Serv_Baseline/vm/linux-i586/client/comp/vm-linux-i586_client_comp_nsk.quick-jdi.testlist2008-01-14-19-43-29/ResultDir/cangetinfo001/
Another failure from nightly:

New MM_REGRESSION failures (from 2008.01.18)
    java/lang/management/RuntimeMXBean/UpTime.java
        This test failed the following assert:

            Internal Error (src/share/vm/code/relocInfo.hpp:1089)
            assert(target != __null,"must not be null")

        on Linux IA32 Client VM (machine colfax003). This is an
        occurrence of the following bug:

            6644928 4/4 Internal Error (src/share/vm/code/relocInfo.hpp:1089)

        I will copy this entry to 6644928.
Another failure from nightly:

New nsk.jdb failures (from 2008.03.08)
* nsk/jdb/where/where005
        This test failed the following assertion:

            Internal Error (src/share/vm/code/relocInfo.hpp:1089)
            Error: assert(target != __null,"must not be null")

        on Linux IA32 Server VM (machine colfax003). This is an
        occurrence of the following bug:

            6644928 4/4 Internal Error (src/share/vm/code/relocInfo.hpp:1089)

        I will copy this entry to 6644928.