Given the dearth of other live examples, I felt like I should post something, though I haven't been able to do a lot of necessary cleanup and polish. It was nice to be able to pull something like this together with just a few hours of work via the Census API, though.
For a given ACS sub-table (anything with an _002E or higher suffix), I'm calling the data for all US counties and dividing those numbers into the applicable total (the _001E figures) for that topic. Which isn't a valid calculation for some of these tables, some some choices crash it entirely — and that's the cleanup I haven't done yet.
But it's still interesting to be able to page through all of this detailed data and spot the biggest concentrations of, say, people who bike to work (http://acsmapper.appspot.com/D3?acs_table=B08006_014E).
I'm really excited about the API, and interested to see what people do with it now that it's opened up.