This morning when I brought up SO and MSO, I was surprised to see the "First time here? Check the FAQ" bar. I checked and this also happened to me on SU and SF, and it also happened on Chrome (so it wasn't like Firefox just tossed its cookies.) I don't think it was just me, because I looked around Meta and found two questions where people mention that they had to login again today. (Neither specifically asked why it happened.)
What was the reason? Was there some change on the server that prompted this? Has the policy of not expiring sessions changed, or was this a one-time thing?
Edit: Some further information which may or may not be relevant: I mainly surf in Firefox, but also in Chrome. I can't remember ever having to login except after intentionally logging out or after clearing browser cookies. I use my own website's URL as my OpenID, it delegates to MyOpenID. I have Google as a backup, but I don't think I've ever used it except maybe to test something on Meta.
I also have an "alt" that I occasionally use to ask questions I'd be embarrassed to ask under my own name (and have linked to my Careers CV). I always use the alt from IE, using a Google open id, and that session is expired too.