The webcams stream by using multipart/x-mixed-replace http push. This happens at least with Webkit 525.x through 528+.
I find this is still the case with 535.14 and on iOS 5.0.1. If you replace the mjpeg to a jpg by setting the src. The image in the webpage is updated to the jpg image but the connection to the mjpeg stays open and continues to stream.
The iPhone does close the connection when a user switches out of the webapp, which is good. But if the user stays in the webapp but is no longer viewing the mjpeg, the connection stays open and bytes are transferred.
I can set up a test case if this would help get this fixed.