iMessage is now officially a registered trademark of Apple. It was officially patented last November 20, 2012, first submitted for review in September of 2011.
The official document for the patent of Apple’s iMessage.
Apple’s iMessage allows users to send texts, photos, videos, contact information, and group messages over Wi-Fi, 3G or LTE to other iOS or OS X users, without any costly data plan required, thus providing an alternative to standard SMS/MMS messaging for all users with devices running iOS 5 or later. This is accessible via the Messages app on an iPhone, iPad or iPod touch that runs on iOS 5 or later or on a Mac running OS X Mountain Lion or later. Owners of these devices can register one or more email addresses with Apple for their use of the newly patented iMessage.