I've tried ustream from my iphone and it was a bit jumpy at home with limited upload speeds; worked much better at my office. We've experimented with ustream for events and it wasn't reliably good enough for us.
You might want to check out twit.tv. They do their broadcasts on ustream, justin.tv, and Bitgravity. I find that their justin.tv broadcasts are quite good, and bitgravity does pretty well, though ustream is a bit fiddly and I often get video without audio.