I have gfortran and gcc 4.3 binaries ready for Leopard using current snapshot GCC sources. Dave has created an installer package for the gfortran binary. This includes separate PowerPC and Intel binaries. Note that this installer does NOT include the XCode plug-in for gfortran.
You can get it from here: MacResearch gfortran Installer
You can also get the gfortran tarball and the gcc 4.3 tarball for Leopard, from my website: hpc.sf.net. Please leave any comments/problems with these binaries here.