> Ned: do you have time to backport this to 3.2 and 2.7?

Yes, I'll be taking care of it shortly.

> Also, the code might not be entirely correct for all machines: on my
> machine 'gcc-4.2' still exists in /usr/bin but doesn't work because the
> actual compiler (/usr/bin/i686-apple-darwin11-gcc-4.2.1) is no longer
> present. This is with Xcode 4.5.2.

AFAIK, the file /usr/bin/gcc-4.2 is not installed with any version of Xcode 4, only Xcode 3.x.  It should have been removed by the command line tools uninstaller that used to be provided in Xcode, prior to the consolidation under Xcode.app in Xcode 4.2(?):

   sudo /Developer/Library/uninstall-devtools --mode=unixdev

Presumably that functionality should now exist in Xcode.app itself.  FWIW, I've not seen /usr/bin/gcc-4.2 left behind with any Xcode 4 installation I've done.