The 'hack' to use 3G data on the iPhone 3G required jailbreaking and then installing either tricker3G or another application (can't recall name VOIP..something, I used tricker3G). Theoretically, the same can now be done on the 3Gs but I've seen numerous reports that tricker3G isn't working and is hosing up Slingplayer Mobile.
This is the only reason I previously jailbroke my 3G so I think I'll wait a bit and let them work out the bugs before jailbreaking my 3Gs.