Wikipedia was down for almost two hours, from 8:36-10:22 UTC.
There was a kernel panic related to the filesystem driver; I'm not entirely clear how this came about, but once that happens it stops liking to talk to the disk, which is obviously somewhat problematic. ;)
The machine is rebooted and running smoothly again (big thanks to our very own Jason Richey!)
-- brion vibber (brion @ pobox.com)