The service is still in closed early testing, but early adopter types can now download it from the Android Marketplace and create an account. Once you have an account you’ll get access to the web version as well. Note that it’s only available for U.S. Android users right now though.
I’ve been testing it for a day now. There’s something compelling about it even in this rough stage. Fourquare fanatics are definitely going to want to try it out.