ASP.NET, by default, has no problems working with HTTPS on 443, the same applies for the RadControls as well. It must be something else that is breaking your pages here. Please inspect the traffic using FireBug or Fiddler and check what is going on with the requests, what are the status codes?
Further, you can check whether port 443 is not the default port for another website. If there is, please remove the mapping on the other site and try again.
You can also check the following help article
on how to set up SSL on IIS7.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed