Yo, I installed OS X 10.6, which apparently defaults to compiling everything for 64 bit, and I'm still running a 32-bit build of Pd-extended. So, trying to compile the latest iemguts gave me "Mach-o, but wrong architecture" when trying to load the now 64-bit compiled externals.
The trick is to add -m32 to the CFLAGS and -arch i386 to the LDFLAGS in the Makefile to force gcc back to 32-bit mode (well, or use a 64-bit Pd but that's for another day).
Hope this helps anyone else on the bloody edge Luke