I would imagine their bullet point about flexibility and direct hardware access is why Rackspace won over EC2. I've used both ends of Rackspace (colo and cloud) and would imagine it would be simple to utilize both services under a common support account (which is Rackspace's big upsell: their support). EC2 doesn't even have a colo option or partner so you'd have to roll your own solution. Better to take advantage of the integration considdering price is close enough.
It's not the world's best but naturally feel free to steal my source and run wild with it.