After the release of mesa 6.5.1 and some patches added to xorg-server, I was able to add compiz to the portage tree. So we now have a running aiglx/compiz-setup in Gentoo.
It comes along with a script compiz-aiglx that runs the neccessary options and loads all default-plugins. If you're using ~x86 and have a card supported by free dri drivers, you should be fine. If you're using stable, a
package.keywords suitable is in my overlay.
If you were using my overlay in the past, make sure you emerge rsync and svn up in the overlay, after that update mesa, xorg-server and compiz from portage. If you relied on the autodetection-hack I previously had, this is removed in the portage-ebuild, so you need to set LIBGL_ALWAYS_INDIRECT=1 and add option --strict-binding (or just use the above mentioned compiz-aiglx script).
Further stuff (configuration tools, quinnstorm stuff) will follow. Submit problems to the Gentoo bugzilla.
Thanks to all the testers and helpers, especially thanks to David Reveman and Kristian Høgsberg for their great coding work.