So I was trying to get udev working on my gentoo install this evening and it was giving me no end of trouble. First when I tried booting it complained that there was no console, you have to look quick to figure this out because the darn thing rebooted pretty quickly. A little digging found the fix for this, udev is not yet started to create the devices needed but the console and null device are needed. To fix this boot to the recovery cd and run the following in the dev directory:
mknod -m660 console c 5 1
mknod -m660 null c 1 3
With this done I thought all would be well. I rebooted and it again broke on me, this time it could not find the entry in dev for the filesystem. So dig some more. I read the howto and forum postings on gentoo.org and followed their setup but to no avail. Eventually I hit upon the problem. Since I was changing over from devfs to udev I had to remove the file in /dev that told the system that devfs was running. The file is .devfs or something like that. Once this was out of the way the system was able to boot properly, creating all the device nodes that were needed. Actually it was just untarring them from a file so I am not running pure udev yet, that will come when I feel brave again.