I ran into similar problems after upgrading to Snow Leopard. I now have the following in my .Profile:
export CFLAGS="-arch i386 -arch x86_64"
export FFLAGS="-arch i386 -arch x86_64"
export LDFLAGS="-Wall -undefined dynamic_lookup -bundle -arch i386 -arch x86_64"
and it works _except_ if I build as su, in which case these variables aren't exported. So I have to remember to build as me, then install as su if necessary.
I don't know a better way to fix it, sorry. But I doubt this is a PyMC-specific problem, so maybe the answer is out there in another mailing list. Please do post here if you figure it out.
Anand