I couldn't get /Applications/Xcode.app/.../usr/bin/gcc to work for me today. It could not access stdlib includes and such. I got the Xcode preferences installer of the command-line tools to work, and that put a gcc in /usr/bin/ that worked completely. I don't know what's different between the two, but that seems to be the official way to do it.

Kudos to Apple for the self-contained command-line tools package download. Hopefully that'll continue with another update for 10.8.