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With regard to your question...as previously mentioned, iMessage only works between iDevices (iPad, iPhone and iPod Touch). The other person also has to have upgraded to iOS 5 on their iDevice. If you get the "not registered" error, that means that all of that criteria has not been met (at least yet). More here:
you can't do it directly on contacts, but go in "mail", type in an email, once a blue bubble forms around it, double tap it until it says, "create existing contact" and then you can add all their stuff, and you can edit by doing exactly what you did to make the contact, instead, select "add to contact". worked for me!
