I do not know what exactly was wrong, but since i updated mesa (on gentoo 9.2.0-r1) and the latest emul-linux-x86 (20131008) everything just runs as it is supposed to.
while sb support was being built by default in Mesa 9.2, this feature wasn't enabled by default (you'd have had to done that yourself). In recent 9.3 dev, sb is now turned on by default. Perhaps the gentoo packagers backported that change, and you picked it up with your update. shrugs.