Instructions for Mac OS 10.7 (Lion) with Xcode 4
As of August 2011, PPC and Intel versions for Leopard are still available on the HPC on Mac OS X sourceforge site. Grab the latest archive of the GNU compilers from the HPC on Mac OS X site.
Below is a bit more detail on the install process.
What's great is that the compilers are pre-built, meaning that all you have to do is download and extract the files to install.
Fortunately, the folks who run the High Performance Computing for Mac OS X website keep recent versions of the GNU compilers available on their site. Xcode 3 includes the GNU C and C++ compilers, but the versions are a bit dated.