According to a Youtube video, the jailbreakers have been able to send messages using the SMS protocol from the iPad 3G.
The 3G iPad uses SIM cards to provide 3G services, which is what the dev team has leveraged for this hack. The iPhone Dev Team for now has demonstrated sending SMS messages using the command line utility in OS X called, Terminal.
The hack is more of a Proof of Concept and we can expect a user-friendly interface. Video: