I haven't put much thought in a solution yet, but at this point I'd go for three changes:

1) Give a clear warning when python was configured for i386/ppc 
   and Xcode4 is installed (instead of giving a vague compiler crash 
   due to crapping out on ppc code)

2) Add an option that forces distutils to compile for i386 only

   This should not be the default because that would result in broken
   packages for users that want to redistribute binaries to ppc 
   machines.

3) The i386/ppc build should use the system headers instead of the
   10.4u SDK (change in configure.in), with a fallback to the 10.4u
   SDK when building extensions on a 10.4 machine (because the 
   "-isysroot /" cannot build universal binaries on all such machines)

BTW. distutils does not have code to work around OSX issues, it has code
to make use of OSX specific compilation features (such as having universal binaries and SDKs). ;-) ;-)