I've been posting on Stack Overflow for a few days with an unregistered (just stored in cookies) account, with the plan to register when I got around to it. I had the impression that this would retain my list of asked questions etc. Anyway, I had the site loaded and just clicked on Questions to see what's new, and the site seems to have forgotten all about me. I didn't shut down the browser or clear cookies, or even navigate off the page, and looking at my stored cookies there are still Stack Overflow ones there, but the site doesn't know who I am. It's not the end of the world, although I do feel a bit bad for the person who responded to my last question and whose answer I didn't get a chance to accept, but I think that if Stack Overflow forgets an unregistered user after a certain number of days, it should be made clear somewhere. I know that when I clicked on register, as an unregistered user, it did warn me that if I were to clear cookies/use another browser, etc., I would be forgotten, but I don't believe it made any mention of a timeout.
PS: I have since registered; otherwise I wouldn't be able to post here.