Many developers using the s3boto backend might also use a CDN for delivering their content. The CDN would be using an S3 bucket as origin.
A simple way of being able to deliver content from a CDN would be to enhance S3BotoStorage.url
My suggestion would be to add two more settings:
USE_CDN is a callback that would be called in S3BotoStorage.url to determine whether the domain should be the CDN or S3.
If you think this would be an useful feature, I will happily open a pull request.