iOS 6 fixes a rare instance issue where a stolen iPhone might remain linked to an Apple ID. A customer’s messages will no longer arrive and get sent from the stolen phone.The bug, affecting iOS 5, would persist even after changing passwords, or after performing a remote wipe and a SIM card deactivation.
The Next Web confirms with an Apple insider that “the issue has been fixed in iOS 6 through a variety of checks placed on iMessage.” Apple now forces customers to re-enter their Apple ID password to use iMessage once their credentials have been changed.
In other words, if someone steals your iPhone, simply change the password of your Apple ID. The new “owner” would need to enter the same password in order to continue receiving and sending your iMessages.