I'm having a persistent repeatable crash that hangs the entire IDE.
During a workspace build, it gets to a step where it's:

"Building all ... - Validation of luceneSearch by XML Validator has started"

at which point the UI is still somewhat responsive but the build stalls, can't be cancelled and Eclipse will not sucessfully shut down (has to be forcibly quit).

I can build the same project fine under XP.

There's not much useful info in the log:

eclipse.buildId=M20050929-0840
java.version=1.4.2_09
java.vendor=Apple Computer, Inc.
BootLoader constants: OS=macosx, ARCH=ppc, WS=carbon, NL=en_US

!ENTRY com.ibm.etools.validation 4 0 2005-12-20 16:50:12.782
!MESSAGE
*** ERROR ***: Tue Dec 20 16:50:12 PST 2005 java.lang.StackOverflowError

Any ideas?

It could be that the installer on Windows sets the JVM to run with more memory and on the Mac it doesn't. Can you update your launch alias on the Mac to provide the following commandline options?
eclipse -vmargs -Duser.language=en -Xms128M -Xmx512M -XX:PermSize=64M -XX:MaxPermSize=128M