I hope you don't mind if I suggest a couple of alterations to this fantastic plugin. One to make it work with the latest structure provided by Rackspace and the other to make the plugin more stable.
The first is to add a field in the settings of the plugin to allow the user to specify the CDN URL. Rackspace now use multiple URLs instead of the old one.
The second is a small tweak, but when using preg_match, to select the required URLs, open and close brackets should be added to the delimiters, which I believe currently consists of " and '. Essentially for when the author of a page has used inline CSS using the style tag and sets a background image using 'background: url(http://example.com)' no more than the url is selected.
I would be very happy to join the development group of your plugin to help make this changes if you don't mind?