Sending SMS in Messages on iPad?

I was really hoping that the messages app in iPad would allow SMS via an SMS gateway (e,g., you could send text to 555-555-5555@txt.att.net) but it doesn't seem to recognize that format as a legitimate recipient. There are third-party apps out there that allow this but it would have been nice if this had just been included in iOS 5. Am I missing something or was this feature left out?