Ok, so sitefinity hasn't yet released a service pack which contains the Q3 2010 controls, however at some point...it will :)
Regardless if you use the telerik suite on your regular aspx pages you've no doubt seen the RadStyleSheetManager control. Putting it on the page allowed you to automatically combine the telerik CSS or embedded in an assembly.
BUT NOW...(drum roll)
It can also combine EXTERNAL CSS! With the caveat of the CSS needs to be living in your webroot.
Here's how you turn it on (thanks to Simon for this)Web.Config: