We haven't made that announcement yet, but we are going to make it.
Anyone who has obtained the source during the last year and played with it a bit, should have noticed an abundance of Sass files located in the Skins project.
Of course, simply using Sass, or Less for that matter, to code stylesheets is not different from hand coding them. The real advantage comes from using proper mixins, flexible variable names and so on and so forth.
As of now, we haven't yet used Sass to its full capabilities, but I guess you can expect to happen so during this year.
More on the topic when we have the 2013 Q1 release and it's blog time.
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