If it sounds like BBM, RIM's BlackBerry Messaging service, that's because, well, it sounds a lot like BBM actually -- and increases device lock-in as well. Users of iPads and iPod touch devices will get built-in text and picture messaging, unlimited and free, which is nice for parents who don't want to splurge on an iPhone for little Timmy, but still want him to be able to keep in touch with friends.
For iPhone users, iMessage will be built in to the standard text message window, so using it should be pretty seamless. Other features include group messaging, delivery and read receipts, typing notification and secure encryption. Of course, keeping with the sync via iCloud theme, users will be able to start a conversation on one device and finish it on another. No word yet if iMessage will be included in iChat for OS X Lion as well, but I wouldn't be surprised if it makes an appearance there as well.