This happened for me after having upgraded to XCode 4; I haven’t had time to figure out what went wrong during the upgrade (or whether this is the intended behaviour), but the following workaround works for me:
sudo env ARCHFLAGS="-arch i386" easy_install whatever
The ARCHFLAGS trick works with setup.py as well:
env ARCHFLAGS="-arch i386 -arch x86_64" python setup.py install