We have been using this technique on the Freshbooks login pages for a while now. (example: http://reason.freshbooks.com) It enables our customers to customize their account colors while maintaining the elegance of rounded corners.
Please. Check the absolutist pedantry at the door. If the technique works and doesn't affect the content management process (which, remember, is the real reason that people want non-semantic markup out of the HTML -- it's so that there's a single place to look for the semantic stuff) then by all means go for it. With just a few lines of jquery you could automate this process based on a single class tag for the "rounded corner" divs or whatnot, and everyone wins.
I'm not completely sold that this is easier than image mangement (it probably is for developers who fear graphics work, less so for designers who fear code hackery), but it's awfully clever to my eyes.
