If you see anything like the following results while compiling a module for python (perhaps even attempting to install a module via setuptools, or a package manager like darwin ports) then you need to get the latest XCode update from Apple, and when doing the install, select the custom install option and make sure to install the Universal SDK. Recent versions of MacPython (2.4.x, 2.5) need this or they will fail to compile added modules.

building 'Crypto.Hash.MD2' extension
creating build/temp.macosx-10.3-fat-2.5
creating build/temp.macosx-10.3-fat-2.5/src
gcc -arch ppc -arch i386 
-isysroot /Developer/SDKs/MacOSX10.4u.sdk 
-fno-strict-aliasing -Wno-long-double -no-cpp-precomp -mno-fused-madd 
-fno-common -dynamic -DNDEBUG -g -O3 -Isrc/ 
-I/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5 -c 
src/MD2.c -o build/temp.macosx-10.3-fat-2.5/src/MD2.o
src/MD2.c:14:20: error: string.h: No such file or directory
In file included from src/MD2.c:15:
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:18:20: 
error: limits.h: No such file or directory
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:21:2: 
error: #error "Something's broken.  UCHAR_MAX should be defined in 
limits.h."
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:25:2: 
error: #error "Python's source code assumes C's unsigned char is an 
8-bit type."
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:32:19: 
error: stdio.h: No such file or directory
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:34:5: 
error: #error "Python.h requires that stdio.h define NULL."
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:39:19: 
src/MD2.c:14:20: error: string.h: No such file or directory
error: errno.h: No such file or directory
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:41:20: 
error: stdlib.h: No such file or directory
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:43:20: 
error: unistd.h: No such file or directory
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:55:20: 
error: assert.h: No such file or directory
In file included from 
/Library/Frameworks/Python.framework/Versions/2.5/include/python2.5/
Python.h:57