The iPad 3G is only a few days on the market, and already dev team members have jailbroken it. We can expect a public release soon.
Meanwhile, MuscleNerd just tweeted he was able to send an SMS using the iPad baseband. He had to use a T-Mobile SIM, but only because the AT&T SIM that comes with the iPad doesn't have an SMS plan attached to it. If it did, there's a solid chance it would send just fine.
This is early on, but a very cool mod - sending SMS from the iPad. I'm betting we'll see a jailbreak software released in the coming future to do this well, if not actually porting MobileSMS.app to work on the iPad. I doubt that'd be the case, since the graphics are only for iPhone. Either way - go jailbreak crowd.