Don't know who Akamai is?  You should.  According to some reports, a majority of internet downloads and audio/video streams run through their content delivery network (CDN).  GigaOm has a solid writeup on Akamai's new HD delivery network for the Web capable of delivering the highest level of commercialized HD - 1080p to the masses.  Realistically, I don't expect we'll see a lot of 1080p content floating around for a matter of years, but certainly within the next five it will become commonplace for commercial  content. 720p will work for a lot of short-form content, with many movie trailers already being offered up in HD on sites including the new Halo 3 HD Mini Movie done with Weta digital (Peter Jackson's outfit).