Most likely it wont. This would seem like a HUGE feature, and probably something Apple would have announced today. But I could be wrong. Just got a registered Developer account today, so still waiting for activation email. Then I'll know.
So it looks like Bluetooth keyboards are not one of those featured in "Most features will be supported by iPhone 3G", unless anyone can confirm them working on their 3G? After watching the Gizmodo video showing it off on a 3GS I can most definitely confirm that it doesn't work like that on mine.