[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).
It also looks like there's a built-in level meter and headphone jack (hopefully with selectable input/output levels...). It comes with two rather robot-like mic arms, which might be nice for recording concerts. I'm presuming one would need to use a Fostex-provided app to record in stereo (I don't think Voice Memos of FiRe, for example, allow two-channel recording).
It's good to see a new iPhone audio interface on the market, since Blue seems to have dropped the ball on iPhone 4 support.
I hope to get my hands on one of these guys to test it and add its information to my comprehensive iPhone 4/iPad 2/iPod Touch external microphone guide.