A CU tried to get and investigate the behavior of GC with -XX:+UseConcMarkSweepGC.
When they start jhat in the hprof file gotten by jvisualvm,
they found a lot of warning message, although eventually web server becomes ready.

Reading from heapdump-1302073897518.hprof...
Dump file created Wed Apr 06 16:11:37 JST 2011
Snapshot read, resolving...
Resolving 27397 objects...
WARNING: Failed to resolve object id 0x9757958 for field TYPE (signature L)
WARNING: Failed to resolve object id 0x99e3008 for field DBG_ON_FIELD_NAME (signature L)
WARNING: Failed to resolve object id 0x99e2fd8 for field DBG_FIELD_NAME (signature L)
WARNING: Failed to resolve object id 0x9e0f070 for field methodSignature (signature L)
WARNING: Failed to resolve object id 0x999b0f0 for field windowFocusListenerK (signature L)
WARNING: Failed to resolve object id 0x999b0b0 for field windowStateListenerK (signature L)
......
WARNING: Failed to resolve object id 0x982a2c0 for field value (signature L)
WARNING: Failed to resolve object id 0x982a280 for field key (signature L)
WARNING: Failed to resolve object id 0x9be7bb0 for field value (signature L)
WARNING: Failed to resolve object id 0x9be66c8 for field key (signature L)
Chasing references, expect 5 dots.....
Eliminating duplicate references.....
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.

CONFIGURATION:
OS : Windows7 untimate(Japanese)/WindowsXP(SP3, japanese)
JDK : JDK6u21/jdk6u24(both are for x86)
jvisualvm : attached to 6u21 and 6u24 respectively

REPRODUCE:
1. Invoke the following command
   java -XX:+UseConcMarkSweepGC -jar jfc\Notepad\Notepad.jar
2. Get heapdump by jvisualvm
 1) Invoke jvisualvm
 2) find Notepad process in the left view pane in visualvm window
 3) click the Notepad process in right button of mouse and
    select "open" in the pull down menu
 4) Click the "Monitor" tab in the right view pane
      and click the "Heap dump" button.
    (heap dump icon will ahows up in the left view)
 5) click the heap dump icon in the above step 4),
      select "Save As" and save the heap dump file some folder.
3. Open the heap dump file in jhat
   ex.
     jhat heapdump-1302152417015.hprof

You will see the above mentioned warning message.

A little bit more details, please see the comment section.