If I were you I would do some searching around at the Gentoo Forums
. They are pretty good at working through things over there. As for your problem, I won't be much help. I few months back I was looking for information on my own kernel panic and ran across a few relating to not mounting root. Most of them recommended verifying that the file systems were compiled into the kernel and not as modules.
That would be the first thing to look at. I also saw a lot of "compiled a new kernel and it worked" which doesn't help much. That is about all I know. Hope you find an answer! Gentoo Rocks!