It'll be a long, long time before the 2010 US Census is released in a format useful to genealogists. But that's not preventing some programmers from creating cool mashups of the demographic data already available. Two such mashups are described at the Google Maps Mania blog. This is much easier with recent census data, since its initial release is already a digital format. When the 1940 US Census is released next year (to the jubilation of genealogists everywhere), it'll still take some time to be indexed.
I'm working on a Google Maps mashup myself, to overcome problems encountered in my research. Trying to find historical places that either no longer exist or the borders around it have changed, is usually a multiple step process. My goal is to be able to search the map by place and year in one step. Right now, I don't plan to incorporate census data, but I may experiment with it once I have a working prototype. It mostly depends if I can get API-level access to a census index. FamilySearch has an API to their database, but I haven't had a chance to look into it yet.