How to automatically deliver Drupal 7's static contents using CDNs?
Easy


Copy the static files to your CDN (Rackspace cloud files in this case)
Install http://drupal.org/project/cdn (drupal 7 requires no core patch)
Configure the CDN module


Update

You can automate the delivery of content files with the File Conveyor mode, this module integrates with the File Conveyor daemon. This allows for much more advanced setups: files can be processed (e.g. optimize images like smush.it, minify CSS with YUI Compressor, minify JS with YUI compressor or Google Closure Compiler, before they are synced and your CDN doesn't have to support Origin Pull, any push method is fine (supported transfer protocols: FTP, Amazon S3, Rackspace CloudFiles). File Conveyor is flexible enough to be used with any CDN, thus it enables you to avoid vendor lock-in.