I was just thinking of using the GC ID for the username essentially, since you can access that if the user is signed in without using the rest of the game center multiplayer stuff, then having it autogenerate a password for your servers. That way it could be possible to set up a way to retrieve the password if you're logged into your game center account after having wiped and restored your phone, or bought a new one, etc. Though I suppose there could be people out there who don't want to create an account for your guys' server that also don't use game center...