PROVIDE was removed from both mesa ebuilds that are currently in the tree: 12 Jul 2006; Donnie Berkholz <dberkholz@gentoo.org>; mesa-6.4.2-r2.ebuild, mesa-6.5-r3.ebuild: Stop PROVIDE-ing new-style virtuals. But without revbumps, it won't affect users that had it already installed... only after I remerged mesa (6.4.2-r2) for some other reason, I noticed "update world" wants to merge virtual/glu. IMHO such inconsistency is not desired?
Found the same situation on my system with media-libs/glut vs virtual/glut (provides removed without revbump). And looking at Changelog of media-libs/freeglut, it seems to have that problem too. Tell me if you handle it too (at least freeglut is maintained also by x11) or you want another bug(s).
Not worth making everyone recompile for an essentially meaningless change, since the change only matters when pulling in deps and not for a proper running system.