My thinking why? Bandwidth vs Screen Size. The bigger the video, the more speed is needed.
On an iPhone, a video feed of 320x320 would fill the screen horizontally, still leaving space below for buttons. And an even smaller video window could be done if a thumbnail of your own video was displayed too in some sort of overlapping rectangles. With some eye-candy around the squares (and having them not overlap), it could likely get to a pretty small video dimension that needed to be sent over the Internet and it would still look good on the iPhone's screen.
AT&T and other cell phone providers might be able to handle that :)
A similar sized video rectangle would look not only look tiny on an iPad screen, it would look silly at that size. WiFi could handle the speeds needed to do a video size that would look reasonable (640x480 or something like that.) AT&T and other cell phone providers I think would struggle to do that.
So that's my predictions!