I run into some problems when I upgraded udev to version 103 on Gentoo.
In order to have it working I needed to:
emerge -C coldplug
emerge -C hotplug
emerge -C hotplug-base
emerge -C udev
rm -rf /etc/hotplug
rm -rf /etc/hotplug.d
rm -rf /etc/udev
emerge udev
emerge baselayout
etc-update
rc-update del coldplug
rm -f /etc/init.d/coldplug
reboot
See http://www.gentoo.org/doc/en/udev-guide.xml and http://webpages.charter.net/decibelshelp/LinuxHelp_UDEVPrimer.html
for more information about udev.