If you're looking to use the iPhone while driving then a Ram Mount works really well. I use mine for GPS or the Clinometer when off road and no amount of bumps has knocked it down.
I use a Mountek mount. Its universal to almost any phone. I use it for my HTC EVO with Otterbox Commuter case and it holds all that bulk better than anything else out there that is not a permanent mount.
Even off-roading I can use my HTC EVO for GPS, Clinometer, and music without all the bouncing around of other mounts for the price. Can your iPhone do all that at once?(shameless jab a iphones:moon
It does use your CD player as a mount point, but who uses CD's anymore?
if you had a 3rd gen you could use the little cubby under the cig lighter and next to the power outlets. mine has a case and fits there perfectly. avoid overly bumpy roads, though.