After going through all the CDN options available for Drupal 6, I wasn't totally satisfied with any of them. My requirements were thus:

• Move user uploaded files to S3
• Rewrite image URL to S3/CloudFront IF the file has been moved
• Imagecache support

The CDN integration module seemed to be the most promising, but I wasn't crazy about patching core plus running a daemon to handle file processing/uploading. Most of the other approaches I saw all assume that the file exists on the CDN, which is an assumption I'd rather not make when dealing with user uploads.

Media Mover!

I was first introduced to Media Mover at the session at DrupalCon DC, and I've been anxious to use it ever since.