After you upgrade, don't forget to rebuild your x11-drivers packages. Use the command that is shown in the elog messages to find out which. In case you missed it:
# qlist -I -C x11-drivers/The output of qlist can be directly passed to emerge like this:
# emerge -1 $(qlist -I -C x11-drivers/)If you are sitting at a gdm/kdm/xdm/... login prompt and can't type anything, press Alt+SysRq+R to switch to raw input mode and Ctrl+Alt+F1 will bring you back to the console.