I finally upgraded to Lion and Xcode 4.3. Also iOS 5.1 on my iPhone 4.
I can happily confirm that FireMonkey apps work great on iOS 5.1 devices.
I already had Snow Leopard and Xcode 4.2 before. All it took was upgrading to Lion and then getting Xcode 4.3. That's it. I did not have to reinstall FPC or FMI at all.