While the full regression testing of all active branches against all of the current compiler versions is in progress, my current (and preliminary) recommendations (despite some test failures):

- for OS X 10.7.x, use the latest released Xcode, currently Xcode 4.2.1, 
and build with clang and debug:

./configure --with-pydebug CC=clang MACOSX_DEPLOYMENT_TARGET=10.7

- for OS X 10.6.x, if possible, continue to use the last released Xcode 
3.2 (3.2.6), which includes Apple gcc-4.2 (/usr/bin/gcc-4.2 not 
llvm-gcc-4.2)

/usr/bin/gcc-4.2 --version
./configure MACOSX_DEPLOYMENT_TARGET=10.6
or
./configure --with-pydebug MACOSX_DEPLOYMENT_TARGET=10.6

- for OS X 10.6.x with Xcode 4 installed (which does not include Apple 
gcc-4.2), use the latest Xcode 4.2 for 10.6 and use clang and debug:

./configure --with-pydebug CC=clang MACOSX_DEPLOYMENT_TARGET=10.6


More results and fixes to follow ...

BTW, the current test failures with clang without pydebug include a 
number of ctypes test failures (in ctypes.test.test_cfuncs.CFunctions).  
If anyone has time to further investigate those, it would be very 
helpful (Issue13370).