This is a great idea for podcasters to off-load both storage and bandwidth issues at a pretty-good-price. Much more reliable than some of the solutions available on the market today. I read somewhere (?) that S3 is using the actual Amazon infrastructure, which makes sense given that Amazon is also doing private-labels for companies like Target.
Check out the comments in the post, Doug Kaye links to his proposal to have S3 function as a CDN. My guess - Amazon will do just this, especially given the ever growing market cap of Akamai (AKAM) which is at 4.64B currently