Check out this hot little gadget, the Fostex AR-4i Audio Interface for iPhone 4/4S (US$119.99) which enables you to record video with stereo sound or use it as a recording device using external mics or line in. Initial reviews indicated that it works pretty well with iPhone 4 but not the iPhone 4S as users reported that some statics are picked up with the iPhone 4S. Hope they can come up with an updated app to solve the problem. You can get it from B&H now and the link is here.
Source – B&H