I\'m using die Facebook SDK 3.1.1 to implement FB Connect in my iOS application. This works fine in the simple case with either the new FB integration (logged in on iOS) or
I had the same problem. Check that your FB App is enabled in Settings -> Facebook. Mine was disabled (even though I don't remember disabling it) and once I enabled it, it was fixed.
In my test process, I've added and removed my FB App several times from my FB Account, which is linked with my iPhone. It may explain why, magically, my app was disabled.