emerge --update world >> Xorg & mouse

You must rebuild all drivers if upgrading from xorg-server 1.3
or earlier, because the ABI changed. If you cannot start X because
of module version mismatch errors, this is your problem.

Т.е нужно перекомпилить

$  eix -IcA x11-drivers
[I] x11-drivers/xf86-input-keyboard (1.2.2@09/23/2007): Keyboard input driver
[I] x11-drivers/xf86-input-mouse (1.2.2-r1@10/03/2007): X.Org driver for mouse input devices