Foursquare just update the latest iPhone application with version 2.2. I have updated 2 days ago.
What so specially about a Foursquare iPhone app update? For a location base services, most users will be using the service through mobile devices.
The new Foursquare application added photo update function which absent for a long time compare with Gowalla. The new application seems execute faster (or they upgrade the backend service as well?)
Foursquare make themselves comparable to Gowalla, as Gowalla makes themselves comparable to Foursquare (Gowalla add in flag venue function earlier).
For Foursquare and Gowalla, mobile platform is their battle field. I believe the battle field can fit in more than 2 players.