Census data is currently protected by a 72-year blanket of anonymity. That means that the 1940 census is ready for public viewing. Of course, what makes this notable is that this is the first time census data has come out since the Internet became so widespread. I've just been over at the National Archive website, trying to track down my grandparents. Unfortunately, the thing seems to be a little glitchy - I couldn't get the site to spit out the actual census records yet. Since they've only been available for a matter of hours, I suppose there's a bit of a rush on at the moment.
Using the census would be tricky anyway, because it's not name-searchable yet. According to one article I saw, that'll be coming in a few months. However, it's supposed to be possible to locate someone's census record now as long as you have their 1940 street address.
I was really into genealogy as a kid, and the appearance of this new resource this morning has tickled that old fancy of mine. 1940 was such a transformative time; I would love to see the records for my family. My grandmother was still living in the same town I would be born in. My grandfather was already in the army by then, so he might be harder to track down. That's my father's side of the family; I confess I don't know where my mother's parents were at the time. I think it's time to talk to my parents and get some more info.