Update: As of the last couple days (11/18/07), an ebuild has been added to portage for these drivers. It’s no longer necessary to create your own.
So the Long Anticipated ATI drivers that support AIGLX are released. I’ve been wishing for this long before anyone ever mentioned it was a possibility. I’ve got my Gentoo machine all set up with the new drivers and working with Compiz-Fusion. Here is the process I used to convert from Xgl and the 8.40.4 driver to the 8.42.3 driver with AIGLX.