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Have you told AT&T you have switched phones? Perhaps it's something they can help you with.
+1. I believe if you call AT&T, and tell them you switched phone, they can deactivate iMessage. Even if you upgraded through AT&T, they might not have deactivated iMessage correctly. But I think they can do it for you.
Also, I think its been reported on here, if you open iTunes on your computer, and delete the iPhone from your iTunes account, it will also deactivate iMessage.