Blur, who are performing a concert in Hyde Park at the close of the Olympics, are going to stream two of their new songs via Twitter on the second of July. This sounds like a good idea to me - it's something that I don't think has been done very often and Twitter has the potential to draw in new viewers very quickly.
On the other hand, I'm not exactly sure why Blur are doing this. As a marketing idea it appears to be part of the current trend to give your music away for free, in order to draw people to live events or merchandise. But Blur have already sold out their Olympic concert, and the only other gigs they have coming up are some smaller warm-up dates, and they don't even have an album about to be released.
It's still a cool thing for them to be doing, but I think it would be far more useful for groups about to release an album and go on tour, especially those without the backing of vast advertising campaigns (or perhaps to replace those campaigns and therefore lower costs). We'll see whether this idea takes off or not, but I can see it being quite useful, and a possible way for Twitter (and Facebook) to make money.
Sky news: Blur To Debut New Songs Live On Twitter