As announced on gentoo-dev, please test and mark gentoo-sources-2.6.28-r5 stable. The two regressions left have been marked in 'Depends on'. One needs further user testing and other is most likely getting kicked upstream.
Stable for HPPA.
And =vanilla-sources-2.6.28.9 arm/ia64/sh/sparc stable
amd64/x86 stable
ppc stable
Closing, ppc64 has newer kernel versions stable at this time.