vB Optimise currently supports Remote Server (manually uploaded files to a remote host), Rackspace Cloud, Amazon S3, Windows Azure and Origin Pull/Mirror.
99% of CDNs are Origin Pull. What this means is that whenever a static resource (like an image, a JS file or a text file) is requested, the CDN will serve the cached resource and queue a check for the "last modified time" of the resource on your server. If the resource has been updated, the CDN will "pull" the content and distribute it across its network, so that the next time that resource is requested, the new version is served.
Really simple, and doesn't require a bunch of semi-proprietary API nonsense like RSCloud, S3 or Azure does. Those CDNs are more complex because they are aimed more at large businesses that need to restrict access to adding/removing files from the CDN.