I was curious at least to get an order of magnitude when it comes to data used by iMessage. So I fired up tshark and watched the "wire" while sending an iMessage to friend. The message itself consisted of "Test" as the subject and the body was "Test message". Bottom line: 1,042 bytes were sent 498 bytes were received. These were actual bytes on the "wire" so EVERYTHING is included. The whole sequence is via https from my iPhone 4 on WiFi and 188.8.131.52 which by the way resolves to nk11p01st-courier014-bz.push.apple.com. Note that the reason I'm calling this test quick and dirty is that it was the third or fourth iMessage that I sent within about 5 minutes. I believe that some of the normal Certificate / key exchange / Handshaking was done on the first message and cached. I didn't see any of that "stuff" during subsequent messages.