High Definition Video Delivery on the Web?

How would you architect and implement an SD and HD internet video delivery system such as the BBC iPlayer or Recast Digital's RDV1. What do you need to consider on top of the Lessons Learned section in the YouTube Architecture post?

How is it possible to compete with the big players like Google? Can you just use a CDN and scale efficiently? Would Amazon's cloud services be a viable platform for high-definition video streaming?