it's a nice idea, but without having optical zoom, it's an awful lot of trouble and expense for basically a good grip and better microphone. I downloaded a 3x digital zoom video program for the iPhone and it seems to work very well. True test will be what the video when zoomed looks like on my HD TV.
if I'm going to carry that, I'm going to put my HD Video camera in my pocket. Now the cost of which is about $400 vs $150 but it's dedicated and has a 42x optical zoom. When holding the Sony in one hand and the iPhone in the other, the weight difference is barely noticeable.
iPhone 4s = 4.9oz
Sony CX160 = 8.8 oz
http://www.pbase.com/timothylauro/image/140905761/original.jpg