Of course the first thing I tried is Youtube on a prim, and it worked (sort of). This opens up many new possibilities in Second Life.
Second test: I made this webcam avatar attachment, a real augmentationist wet dream. It is very simple, open a ustream account and assign one of the media stream feeds to a face of a prim. Here I used the pop-up window feed: http://www.ustream.tv/channel-popup/giulioprisco