Mesa 7.9 was just added to the portage tree. Along with it came a new eselect module, eselect-mesa, which allows you to switch between classic and gallium drivers.
Especially interesting are the Radeon r300 and software renderer gallium drivers, which can give a significant performance and feature boost over the classic counterparts (enable USE="llvm" for best results). Gallium hardware drivers require DRI2/KMS to be active.
Please give it a good testing and report any issues that you find.