Greg Kumparak explains the iMessage portion of the latest whitepaper on iOS security:
To over simplify it: imagine you have a mail box. This box has two keys. One key lets you drop mail into the mail slot, and one key lets you take mail out. The input key and the pickup key are entirely different; one can never be used to replace the other. You can give away a million copies of your input key, and no one could use it to do anything but put mail in. Unless they find a copy of your pickup key or find a weakness in the way your mailbox was designed, your message is safe.