iMessage is data only. Even though you can send to another phone number, Apple's servers know that the other end receiving that data is another iDevice, unless of course you disable iMessage, then it goes through as a text message, and I say iDevice, because I iMessage from my iPad all the time using my phone number and the person on the other end never knows it. You can iMessage from an iPad, iPhone, iPod, and they use data and the data is encrypted through Apple's servers. The feds are pissed because they have to get a warrant to view the data and even if they get a warrant, from what I read online, there can be a 4 to 6 month waiting period.
Anyways, nothing to worry about, an iMessage is strictly encrypted data, sort of like an iVPN so to speak lol.