As long as it is A2DP compliant it shouldn't matter. I stream Pandora from my Vibrant to my current Pioneer head-unit (don't have model# handy sorry) without any major issues. A quick Google search shows that the iPhone got full support for the A2DP profile as of version 3.0
My only complaint is that if anything mucks with the connection Pandora will just freak out and stop, rather than switching to the phone's speaker. Such as ..... stalling at a light and restarting the engine (kills the head-unit and nukes the Bluetooth connection). This also has a 50/50 chance of causing my phone to stop considering the head-unit a valid output device and I have to re-enable media support in its settings (from the phone). This takes like 4 seconds, but isn't fun to do while driving. I'm considering just forgoing it and getting a 3.5mm cable and plugging into the aux port.