Indeed, it's the known bug in 2.6.21 as mentioned on the debian lists: http://lists.debian.org/debian-user/2009/01/msg00006.html

So now I know, thanks slashdot!

I'm a sysadmin for a small data centre.

This morning, at about 7am UTC 10 machines running Fedora Core 7 all locked up. They were on a private network inaccessible to the general internet. The lockups were all complete: not even Caps Lock on a connected keyboard worked (a good sign of kernel death). Rebooting and checking the logs gave no information: the last syslog entry was sometime after midnight and was normal. Once restarted (with a hard reset) they all seem to be functioning normally. I don't know yet what caused the lockup.

It's as if a million Fedora boxes cried out in terror and were suddenly silenced...