To map a domain to an S3 bucket:
1. Choose a domain name, and on your registrar's site, make it a CNAME for s3.amazonaws.com. For this example I've chosen static.reallysimple.org.
2. Using the AWS Management Console, create a bucket with the same name.
3. Store stuff in the new bucket. You can use the name defined in step 1 to refer to the contents of the bucket created in step 2. Here's the url of a file in my bucket.
That's all there is to it. It's really simple. Amazon did this in a very straightforward and elegant way.
Why would you want to do this? It's neater, and it gives you flexibility down the road if you want to move the content somewhere other than S3.