Yeah, I know I should find some other StackExchange site, go there, create an account there, login there, and post this complaint there - but you know, that's just more of the same problem. </rant>
Please note we did force logout globally, network-wide in April 2010 (to mandate emails for Google GMail OpenIDs) and September 10th 2010 (to enable global auth for all users).
Also, we delete any sessions older than 4 months as a matter of course.
So if you visit VERY infrequently, you might run into one of those.
(They may have fixed this recently, but it was like that for the longest time.)