Another machine.
> Is MACOSX_DEPLOYMENT_TARGET set in the environment when you run the command that gives the error message?
I don't think I had this environment set when I saw the above error 
message. I had to set MACOSX_DEPLOYMENT_TARGET=10.5 in order to 
workaround it though.

BTW, I just figured that following command will reliably reproduce this 
issue:

$ MACOSX_DEPLOYMENT_TARGET=10.3 python2.7 -B -s -c "import 
sys;print('%d.%d' % tuple(sys.version_info)[:2])"
Traceback (most recent call last):
   File 
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", 
line 558, in <module>
     main()
   File 
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", 
line 540, in main
     known_paths = addusersitepackages(known_paths)
   File 
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", 
line 264, in addusersitepackages
     user_site = getusersitepackages()
   File 
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", 
line 239, in getusersitepackages
     user_base = getuserbase() # this will also set USER_BASE
   File 
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site.py", 
line 229, in getuserbase
     USER_BASE = get_config_var('userbase')
   File 
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sysconfig.py", 
line 518, in get_config_var
     return get_config_vars().get(name)
   File 
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sysconfig.py", 
line 421, in get_config_vars
     _init_posix(_CONFIG_VARS)
   File 
"/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/sysconfig.py", 
line 300, in _init_posix
     raise IOError(msg)
IOError: $MACOSX_DEPLOYMENT_TARGET mismatch: now "10.3" but "10.5" 
during configure

Though, I do recall seeing this error without having that environment 
set at all.