A new mesa release candidate is out - using the rc3 ebuild worked for me. This is my first bugzilla submission, hope everything's correct. ;)
http://www.mail-archive.com/mesa3d-dev@lists.sourceforge.net/msg05398.html
http://www.mail-archive.com/mesa3d-dev@lists.sourceforge.net/msg05497.html "Long story short: Mesa 7.1 requires xserver > 1.4.99.906"
"*mesa-7.1 (06 Sep 2008) 06 Sep 2008; Donnie Berkholz <dberkholz@gentoo.org>; +mesa-7.1.ebuild: Version bump. " http://sources.gentoo.org/viewcvs.py/gentoo-x86/media-libs/mesa/ChangeLog?view=markup this bug should be fixed then?