Register on iMessage : How To!! - It appears that the person you want to message has to have an apple id, and using the cell number or email in that apple id account, if not I don't think you can text them, also they need to be on a iOS5 device.
Remember that on iPad iMessage only communicates with other iPad, iPhone and iPod Touch devices. It will not message with your entire contact list. If you want a messaging app that will text to anyone, you could download Text Free from the App Store.
I guessed about a couple friends, presuming them to have iPhones and early updaters to iOS5. Sure enough they showed as legitimate to iMessage (no red exclamation point). Mine & daughter's iPhones legit as well after iOS5. Messaging to my new me.com e mail address works to/from my iPad 2 & iPhone. Message to my aol e mail address went to iPhone only. I guess as iOS5 proliferates I'll see less red exclamation points.
