The Android and BlackBerry versions are still available, but the iPhone is by far the most popular platform for Foursquare users. This is just a temporary technical setback, though. Foursquare tweeted the following.
Earlier this week, the new 1.6 version of the Foursquare iPhone app was accidentally added to the App Store before it was ready. The problem was corrected, but now people who have 1.6 are unable to use the service. They're being directed to the app store to download the regular version, but it's not there.
Foursquare is in the middle of unveiling new features and deals so these technical issues could have come at a better time, but it won't be a major setback. Don't worry, Foursquare fans; you can keep looking forward to the new features in version 1.6 this week.