Of course, there is that pesky problem of writing files to disk... Luckily there are signed scripts.
P2P is everywhere these days, in many many things. The ability to do these things directly in the browser without having to rely on a separate executable should make things much simpler, and better too!
I'm not talking about illegal downloads either. ;) I wouldn't be surprised if in 5 years every download, for every-thing, was a P2P download with a HTTP-seeder. It would save many web-hosts tons of bandwidth and dollars. It just makes sense from their perspective. The only nail in the coffin would be the lack of net neutrality... darn.