a little while ago i remember seeing some posts from people whose machine hung up completely and displayed a frowny icon along with a message "oh no..." saying some sort of severe unrecoverable error had occurred. i did not have the problem at the time, but yesterday i did a full pacman -Syu (after a week off for the holidays) and upon reboot it happened to me. as it turned out, the fix was to do nothing whatsoever, except not make an error when entering my password at the gdm login screen. i.e. - upon boot, if i enter the password wrong once and then enter it correctly the system ends up hanging. if i enter it correctly the first time there is no issue and the system works as it used to. i am far too busy at the moment to dig around right now but perhaps this is a clue to someone that deals with gdm that there is still a problem of some sort lurking around, and the usage scenario might help them zero in on it.