Hey I was thinking about this sort of thing a while back. Great job. A couple of comments/questions:
1) Your test video seemed to have a lot of stuff with you being stationary and maybe doing some panning, but to really show this off I'd like to see some samples of you running up/down stairs with it, etc.
2) Is the gimbal just needed for the (optional) handle? If I can live without a handle, do I just have a single solid bar connected to the iPhone with the two counterweights at the bottom? If it's needed regardless of the handle, how would you hold it? Above or below the gimbal? I think I've seen other DIY steadicams (not for the iPhone) which didn't seem to have a gimbal. Example:
$14 Steadycam The Poor Mans Steadicam