I know this topic is a little old, but I found it earlier today whilst trying to find a way to get UStream videos to play on my RaspBMC installation. I stumbled upon this:
It's from the same developer as the "Livestreams" plugin, which I couldn't get to work for the life of me; but I installed this, and it seems to play (and find) most streams that I want. Though a word of advice: If you try this, be sure to change "Stream type" to "Choose" in the settings to save yourself a headache. It defaults to one specific stream type and instantly fails with it's default settings
Hope this helps!