Hey all -- so instead of looking around to see if there was already an active Gentoo RPi community, I foolishly went and did a bunch of the legwork myself. At any rate, I thought I'd share in case this is useful to anyone:
Basically you just run the script, and in the end (after a *very* long wait), you get a SD card image you can write out to an SD card to boot Gentoo. It's not perfect, and updates to packages in Portage might break it, but I've successfully built a few images using it.
I also didn't know about media-libs/raspberrypi-userland, and have included my own sys-libs/rpi-userland. It can build either from the 'master' or 'next' branch of the raspberrypi/userland repo on GitHub. If you choose to build the experimental stuff, it'll also build the 3.6.x kernel instead of the 'stable' 3.2.x kernel.
One of the things I've definitely done differently from the official raspberypi-userland ebuild is I install all the libraries and headers to /usr instead of /opt/vc. I really don't see a reason to use /opt/vc, esp since it's all built from source. The GL (etc.) libraries are also all managed with eselect-opengl (though there's a hack in there because eselect-opengl freaks out if there's no libGL.so, which of course we don't have on the RPi).
There's a fairly extensive README to get you started if you want to play with it.
Anyway, as I said, I hope people find it useful... at the very least, I learned a bit about how the RPi is set up by messing around with it.