I run the gentoo-dev-sources-2.6.8 kernel successfully on a Sun-Sparc Ultra5 workstation. I had to manipulate the ebuild file to emerge the kernel sources. Also the r1, r3, r3, r4 releases are missing the keyword.
No. Our patchset, which is required for 2.6.x kernels to work properly, isn't included. 2.6.8* also has several other nasty bugs on sparc. If you want a 2.6 kernel, stick with non-gentoo 2.6.7 and apply our patchset.