When I was [writing about Apple’s privacy statement][1] regarding PRISM, I noted that wording around iMessage was rather vague and that I wanted to know more. I really didn’t get more information, but cryptographer [Matthew Green was also interested and did a little poking around iMessage][2]. His post is a fantastic read about what Apple may or may not being doing — because really we need Apple to reveal at least the high-level methods of encryption they are using before we know any of this for sure.
What does seem clear from Green is that iMessage *is* encrypted, but that Apple controls that encryption — this is an important point. It seems likely Apple *could* turn over at least a few days worth of iMessage content if compelled by a Government order — and highly probably that they store the meta-data for iMessage in a way that is easy to turn over if compelled.
Essentially it comes down to: how much do you trust Apple, and how much do you care about the government reading your iMessages?
[1]: https://brooksreview.net/2013/06/apples-commitment-to-customer-privacy/
[2]: http://blog.cryptographyengineering.com/2013/06/can-apple-read-your-imessages.html?m=1