Chas, you probably need to rebuild python with a bigger default thread stack size. You have to code a bit to do it; it involves modifying the thread initialization code. It isn't tough, I dont know if anyone has packaged up a patch for it. You can check the zope-dev mail archives for details.
Thanks for the speedy response, Matt. Since I'm just migrating one of my zope sites to a new server and won't be doing future development on it, I think I'll just roll back to a previous version of Zope. It's not that I'm a sloth but it's already 3am and I was hoping (perhaps optimistically) to get this done tonight. ... that was 6 hrs ago. Don't wish to waste anybody's time so will roll back. Really do appreciate the fast diagnosis, advice and response though. Cheers. chas