Just to make sure other readers aren't confused by different info in this thread; these 2 posts are wrong.
It is not just iOS to iOS nor iPhone to iPhone. It works between devices running iOS 5.
Also, it is NOTHING like BBM...NOTHING. It's simply an added feature within the existing messaging app. Not sure what you mean by "..allows more options" either. Like I just said, it's just added features to the messaging app. It shows users "delivered" and "read" (if recipient has that turned on), and the text bubbles are white and blue instead of white and green. It's nice to have if you have a poor cell signal but a wifi connection...but that really only is beneficial when communicating with other iOS 5 users.
BBM is a completely separate app on BlackBerry's with a completely different UI from the regular messaging app. Whatsapp is similar to BBM.