This was actually an enjoyable read. The best part for me was making good on the promise they made with BetaList, having successfully featured Pinterest, Mailbox, IFTTT, and so on.
I found a comment on this blog [1] by someone claiming to have reverse engineered the iMessage protocol. [2]
In his comment, he says: "OS X 10.9 (and probably also iOS 7) does certificate pinning for the push connection, I don't know about the directory lookup."