Okay, I've noticed that since upgrading to Panther, my VM crashes at 
least once daily.  Even updating to 10.3.1 last night has not resolved 
the problem.  I've not yet dug deep into the source for my VM 
(generated from VMMaker sources 1+ months ago), but here's a stack 
trace of the offending thread from the Mac OS X Console log viewer:

Thread 2 Crashed:
  #0   0x935e2170 in shape_union (shape_union + 748)
  #1   0x935e617c in CGSUnionRegion (CGSUnionRegion + 216)
  #2   0x91577a00 in AddThisGlobalRgnToDirtyRegion 
(AddThisGlobalRgnToDirtyRegion + 84)
  #3   0x91575324 in AddRgnToDirtyRegion (AddRgnToDirtyRegion + 228)
  #4   0x9156c594 in CommonBits (CommonBits + 396)
  #5   0x91574338 in StdBits (StdBits + 692)
  #6   0x9156c620 in CommonBits (CommonBits + 536)
  #7   0x915767cc in CopyBits (CopyBits + 876)
  #8   0x00065694 in ioShowDisplay (ioShowDisplay + 536)
  #9   0x000a9a70 in displayBitsOfLeftTopRightBottom 
(displayBitsOfLeftTopRightBottom + 472)
  #10  0x000ba160 in primitiveShowDisplayRect (primitiveShowDisplayRect 
+ 136)
  #11  0x000b91a0 in primitiveResponse (primitiveResponse + 5732)
  #12  0x000ad4c8 in interpret (interpret + 5908)
  #13  0x900247e8 in _pthread_body (_pthread_body + 40)

Frames #2-4 are common to most of the crashes I've been seeing, which I 
believe is a different location than what John McIntosh was 
troubleshooting.  AFAICT, frames #0-7 are in the OS, not in Squeak, but 
I'm not overly familiar with this part of VM source code (well, any of 
it, really).  Xcode turned up no references to those symbols in the VM 
source.

Could be an OS issue, although perhaps the semantics of some calls to 
the OS have changed.  Anyway, FYI.