I think it's going to depend on the APN the new iPhone has factory set to connect to the internet. I can tell you this, however. I took the micro-SIM out of my iPad 3G (with an unlimited plan), and installed it into my iPhone 3GS with a micro-SIM adapter, and it "could not authenticate on the cellular network" (I forget the exact wording of the pop-up message). Therefore I couldn't use any kind of cellular data with my iPad SIM in an iPhone 3GS. I do know that the iPad uses a different APN than the iPhone, "broadband" APN (for the iPad) versus "wap.cingular" (for the iPhone), and on a stock, non-jailbroken AT&T iPhone, the settings to change the APN are disabled. So unless you can change the APN settings on the iPhone 4, I doubt it'll work.