Important!
You have to 'unregister your account' in the app before restoring.
Then you have to use the same user ID when you register on your freshly installed iOS.
If you have already restored I've got a feeling you have to email pingchat's support and they will do the un-registering for you. http://www.pingchat.com/contact/
Feel free to correct me if I am wrong because I have not done this yet - waiting for greenpois0n tomorrow
Thanks. I have no idea how to un-register my account. I'll email them.