Comes from this thread on comp.lang.python:
http://groups.google.com/group/comp.lang.python/browse_thread/thread/e15915df2e9cdd9/

The Makefile for distutils in the Mac binary for Python 3.1 specifies
the use of the OS X 10.4u SDK. However, XCode only includes the SDK for
the newest two versions of OS X, currently 10.5 and 10.6. This causes
build problems with gmpy on OS X Snow Leopard and the newest XCode. It
probably will also cause problems on other libraries that are compiled
using distutils but I've seen no other reports of issues. I don't know
enough about makefiles to come up with a patch that would work on all
supported versions of OS X. 

The offending code is line 118 in the Makefile, which installs itself in
/Library/Frameworks/Python.framework/Versions/3.1/lib/python3.1/config/Makefile.
The line is
UNIVERSALSDK=/Developer/SDKs/MacOSX10.4u.sdk

Here's a copy of Mensanator's errors:

$ /Library/Frameworks/Python.framework/Versions/3.1/bin/python3
setup.py install
running install
running build
running build_ext
building 'gmpy' extension
creating build/temp.macosx-10.3-fat-3.1
creating build/temp.macosx-10.3-fat-3.1/src
Compiling with an SDK that doesn't seem to exist: /Developer/SDKs/
MacOSX10.4u.sdk
Please check your Xcode installation
gcc -arch ppc -arch i386 -isysroot /Developer/SDKs/MacOSX10.4u.sdk -
fno-strict-aliasing -fno-common -dynamic -DNDEBUG -g -O3 -I./src -I/
opt/local/include -I/Library/Frameworks/Python.framework/Versions/3.1/
include/python3.1 -c src/gmpy.c -o build/temp.macosx-10.3-fat-3.1/src/
gmpy.o
In file included from src/gmpy.c:206:
/Library/Frameworks/Python.framework/Versions/3.1/include/python3.1/
Python.h:11:20: error: limits.h: No such file or directory
/Library/Frameworks/Python.framework/Versions/3.1/include/python3.1/
Python.h:14:2: error: #error "Something's broken.  UCHAR_MAX should be
defined in limits.h."
/Library/Frameworks/Python.framework/Versions/3.1/include/python3.1/
Python.h:18: