updated 01:00 pm EDT, Wed October 12, 2011
Foursquare Radar goes live for iOS 5 first
Foursquare put the iPhone back at the front of its app priorities with an update to its iOS app (App Store). Radar uses iOS 5-specific background location features to automatically notify a user when close to a given location. It can at a minimum send a notice when friends have recently checked in nearby, but it will also flag unused locations on a to-do list to remind users when they're close.
iOS 4 users get more sharing options and a new "me" profile.
The update is available immediately and doesn't have iPad optimizations. Foursquare hasn't said if or when Radar would propagate to Android or other platforms, but they would need to use very different program calls to work.