In Drupal 7, one should use file_create_url() to create a URL to link to a file. This single function should be used to generate all file URLs, so that all file URLs can be altered to use a CDN instead (or your own static file servers, or whatever you'd like).
However, the CDN module already supports file_create_url() in the form of a Drupal 6 core patch. Just today, somebody created an issue, asking (explaining incorrectly) how to make Fusion themes compatible with the CDN module (i.e. how to make it use file_create_url()). Well then, I thought, why not roll a patch so that any Fusion-based theme automatically does the right thing and would therefore automatically be compatible with CDN integration?
The added advantage is that you just have to delete a couple of lines of code and it will be compatible with Drupal 7 right away!
So, attached is the patch that does just that. It automatically detects whether file_create_url() is available and falls back to the "Old Drupal 7 Way" if it's not available. The patch has not been tested, but the code should be right.