A good session with ElectricFence solved the problems I was having everwhere except on OS X. The problem there seems to be that the Objective C runtime is somewhat different than the GCC runtime, even though they use GCC. Go figure.
I may drop support for Mac OS X in ObjCI altogether if that's the case -- I have no plans to use Mac OS X once Apple's Intel migration is complete and the FreeBSD PPC port is released.
At any rate, a search for ObjCI on Freshmeat will turn up the project page there, if anyone is interested.