geocoding - API to reverse geocode latitude, longitude to census tract?

I have a bunch of latitude,longitude coordinates (mostly city/town centroids) in the United States for which I'd like to find the containing census tract.

For instance,

input => nn.nnn, -nn.nnn 
output => 010300

Is there a free or low-cost API out there that can help? (Essentially Yahoo Placefinder, with the additional response of census tract.)

(I understand that many cities include multiple census tracts and I'd expect that some small towns contain only parts of one or more census tracts. I'm comfortable ignoring that issue for now.)

Answer

The FCC census block conversion API is exactly what you're looking for.