I'm seeing the same thing here.
Kernel 2.6.15-gentoo-r1, Nvidia 6800GT on AGP. This video card and my kernel config worked for 2.611. Needed to upgrade due to clean up for support of udev after last baselayout upgrade.
I've run nothing but Nvidia cards for about 6 years. I've been running Gentoo for most of that time. I've
never had this much trouble getting X up. I'm not posting any info beyond that because, frankly, I've tried so many changes to my kernel config and xorg.conf that I'm not even sure where to start.
Yes, I'm running 'emerge nvidia-kernel' after every kernel rebuild. I've also tried doing that, 'emerge nvidia-glx' and 'opengl-update nvidia' as well after a rebuild. I've tried manually adding the nvidia module using modprobe. I've tried simply rebooting and letting it load on its own.The closest I've gotten shows me something very, very strange. Nvidia module loaded. startx seems to start normally, but there's a black screen. reboot/kdm startup; same thing.
Log in through ssh and run "glxinfo". Shows it's running Indirect, MesaGL! I don't have any Mesa software anywhere on my HD, nor is it compiled in or as a loadable module in my kernel!
Now, THAT seems juuust a tad strange to me. I can only conclude that some interaction between 2.6.15-gentoo-r1, the latest baselayout, and the nvidia drivers has broken down. I don't have a clue to know where to begin troubleshooting it.