A few months ago, a scandal started involving iOS 5 and the iMessage system. Then it was discovered that iMessages reach stolen or sold iPhones, even if users disabled the system on them, replaced the cards or changed the password of the Apple ID used with the system. Apple has changed the way iMessage works, so that now a change in the Apple ID password requires a relogin for iMessage and changing the card deactivates the system on the iPhone you removed it from.
Apple did not make this information public, but sources from within the company did confirms the implementation of those changes in iOS 6 and Apple's servers, so now we should have a much more secure iMessage. From the date of the appearance of the said scandal and until now, there have been no other user complaints regarding the way iMessage works on stolen or sold terminals, so the information presented previously turns out to be false for the time being.
