I've had some people send me e-mails over the last couple of months on how to use my RoundedCornersExtender. I would usually give them a half-baked answer from the bowels of my memory and apologize for not putting more thought into it.
But thanks to the persistent Dave Hobbs (http://www.licenseprofessor.com), I now have an updated version on the control. It now works with the latest versions of Atlas and the control toolkit (sorry, can never remember the official names). And it is bundled in a separate project so there is no need to copy files into the control toolkit. The control is named RoundedCornersEx so as not to conflict with the existing one.
NOTE: The project has a project reference to the toolkit project. You'll need to update that reference to make it work. Apologies for that manual step but the toolkit is just a zip file so I have no idea where it may be installed on your system.
Also note that there is some work underway to incorporate these changes into the official build. Have no idea when that will happen though. It's not in the latest release in any case.
** UPDATE ** Latest version of the toolkit now has this functionality built-in and implemented better than mine. Download mine at your own peril.