Check into Foursquare without an active data connection.
Two use cases are:
You want to check-in but want someone else to do it as you are attending Occupy something something and don't want to reveal your immediate location to authorities.
Cache the venues and check-in from the offline wilderness. You and your party continue the hike while your buddy checks in for everyone when he/she gets back to civilization.
Uses an existing open source peer-to-peer via bluetooth android app called Splinternet and grafting on a Foursquare activity. Uses the oauth tokens and venue id to check in for someone else.