Some details to add to what Marc said:
>I've had 3 random freezes of the keyboard upon startx, with the monitor
>going black and the only way out a hard reboot. I've fiddled about with
>fsck (and been yelled at by German geeks for doing that) but something
>seems to always work to get it back. Not the same thing every time, tho.
Did you tried the SUBmarine tactic? Alt+SysReq+S (sync disks), wait until
LED and sound stop, Alt+SysReq+U (umount disks, remount read only), wait
until LED and sound stop, Alt+SysReq+B (reboot). The machine goes down quite
fast, and a lot safer than drilling holes in the ships hull (aka hit the
reset button), cos it can come up perfectly. Of course, all this works only
if the kernel is still alive.