iMessage seems to be a permanent staple for the new iPhones on the iOS 5. Maybe even a little too permanent.
If you lose your iPhone or have it stolen there are a lot of things that you can do to make sure that no one has access to your private material, or the ability to contact any of your family or friends through your line of service. You can do things like wiping the data remotely, removing or changing your Apple ID password information, switching around numbers, not to mention the security and passcode settings that you can place on the phone ahead of time that can create a “self destruct” mode that deletes all phone information if the wrong password is entered in a certain number of times. Unfortunately, none of this seems to affect the iMessage system on the iPhone.
Stolen or compromised iPhones seem to have no protection against allowing someone else to message your contacts using the Apple specific iMessage system. This new element, which was brought about with the iOS 5, allows for a free, Internet based, text message service between devices that are running on the iOS 5, which can include iPod Touch and iPad models. This is similar to BBM on the Blackberry device line. Though it would seem easy enough to stop the availability to use iMessage to your contacts from your device once it is no longer in your possession, users are finding that people who have stolen their phones are still iMessaging people from the device. Even once the device has been re-registered and a new account is applied the same ID and contacts remain the same.
This is an issue that Apple is going to have to deal with quickly, and it serves to reason that they would since an exploit like this is only left open since it is such a new service. Unfortunately, since the problem is likely at the account level you can expect more than just an iOS update to fix it and we may be waiting for quite a while.