[skwlsout]

Will be dependent on the strength of the 3g signal, and whether the ustream app is intelligent enough to send adaptive video from your phone(i.e. does it downgrade/upgrade the video quality automatically depending on the signal strength)

As long as the iphone is getting it's web connection from the 3g and not another wifi point, then as they will be using different internal antenna's there should be no problem tethering other devices to the iphone, (assuming your phone network provider allows tethering)