[Update: I received a demo unit and put it through its paces: read my review of the Fostex AR-4i here. (Works with both iPhone 4 and 4S).]
Just found out about this new product through the YouTube grapevine:
The Fostex AR-4i is said to allow for multiple microphone input, and works with the iPhone 4's dock connector. It looks like a rather klunky device, meant more for handheld use than any existing tripod mount (unless the device has a tripod mount on it, which would be handy).