Chief Architect

There's an excellent feature here on how YouTube built and scaled their infrastructure (thanks, Pete). The general rule seems to be to keep things simple and throw cheap hardware in to scale - oh, and use third party CDNs for the heavy loads.

Scaling http serving isn't rocket science, as this article outlines and it should be noted that progressively downloading 250Kbps short clips is a very different animal to effectively streaming scalable 2Mbps streams (something which companies such as the major CDN companies and Narrowstep are very good at).