Looks like we/somebody coded an overflow into our cookie generator, so some time over the past 24 hours, all cookies were reset from 2038 to 1901 - which means they promptly expired after the browser session using them ended.
If you find yourself logged out but don't remember logging out, don't worry - that was our fault and nothing nefarious has happened. (If you were one of those people permanently logged in, like me, and now find that you can't remember your password, you can email or IM me and we'll get you a reset.)
I've changed the cookie expiration date from a relative time to a static one. I have no idea why it was written this way but we probably inherited it with the code in 2001 and never bothered to notice.
EDIT: This probably happened Monday-ish, but you'd have to clear your existing cookie (that is, log out) to have gotten a new cookie experiencing the Y2038 bug. This is probably way more than anybody needs to know.
The Mediapartners-Google bot picked up the megaclick. Closest users on either side were Broncolanche and Leroy. Recent millions (and how long it took to get there) 57M 14 Dec 2009 16:17:10 (54:00:07:08) 56M 21 Oct 2009 17:09:32 (59:04:38:04)