Uh oh, what will happen to the Gowalla api?
If you're looking for a check-in API, Foursquare's seems like the best bet at this point. The Venues (ie. places) Platform is especially good, with rate limits starting at 5,000/hr: https://developer.foursquare.com/overview/venues.html
There are other good APIs to consider as well: Google Places, Factual, Yelp, etc. Each API has its own pros and cons in terms of query limits, terms and restrictions, fees, etc.
This is a good starting place for research: http://www.quora.com/What-are-the-pros-and-cons-of-each-Plac...