Please note that this is only available for jailbroken devices.
If you've seen Facebook Home on Android, you'll have seen the Chat Heads feature which allows you to message any of your Facebook friends from anywhere on your Android device. A little icon with your Facebook messages simply floats on top of everything else and can be moved to anywhere on the screen.
The Chat Heads feature did come over to iOS, but with the limitations iOS has, it doesn't work just anywhere. It's limited to the Facebook app.
However there is now a Cydia tweak to fix all that. MessageBox is a Cydia tweak which allows you to have Chat Heads anywhere you like and it works like a charm. It's completely free and should be available on the BigBoss repo.