It seems to me that we ought to be able to have both gcc 2.x and gcc3 installed, since other distributions manage to do it (using kgcc for the 2.x gcc, or a similar nomenclature).
Okay, I think the way to accomplish this feat is to echo Mandrake: Change all gcc ebuilds to build as gcc-${PV} using --program-suffix=${PV} (and installing into /usr/bin). Then have /usr/bin/gcc, /usr/bin/g++, etc be symbolic links to the preferred (meaning most recently installed) binaries. I'm pretty sure this should work, although I haven't had a chance to test it yet. As a complete non-sequitar, need to check that our gcc ebuilds use --with-system-zlib to take are of the recent security debacle.
This turned out to be a real pain. It's going to be easier to patch current packages to work w/ gcc3 instead. Expect a gcc3.1-enabled version of Gentoo soon.