Lame… I recently sold my iPhone via Gazelle.com (got a decent amount for it, damn near paid for the iPhone 4S that replaced it.)
Turns out now I have to worry about the new owner receiving my messages? Bottom line, iMessage privacy is completely broken, and no word yet on when (or even whether) Apple plans to fix it. You can turn off iMessage and just send and receive text messages– you lose some functionality, but it is the best course of action availabale at this point if you have recently sold your iOS device (or if you’ve recently lost or had one stolen.)