I don't think youtube will open up the live stream feature to the masses, where anyone can live stream, like ustream or JTV because their infrastructure can't handle it and honestly most of the stuff regular people produce are horrible and a waste of resources.
They will in the short run offer live streaming to content partners and special people (celebrities) that produce content that people actually want to watch and has production value.
Ustream is known for having celebrities on their platforms, but once Youtube live stream platform comes out they will be gone. No celebrity will be able to resist the amount of eyeballs that youtube can bring and the switching cost is very low. It would be safe to say that people who have Ustream accounts also have Youtube accounts and Ustream hasn't signed any exclusive deals with celebrities.
Youtube will be the distribution platform of the premium live streaming content and the rest will be for the low brow stuff and illegal content.