A couple of days ago Foursquare's iPhone application disappeared from the iPhone App Store due to a "hiccup." If you doubted that was true, worry no more, as the application is back in the store.
Here's the official explanation of what happened:
"Last week, Foursquare accidentally released to the App Store a new version of its app before it was totally ready. It was pulled shortly after, unfortunately, many users had already went through the update, and were left stuck with a non-functioning app."