Just a heads up that this issue seems to have made its way into 12.11 beta 11, and possibly other versions. I've had it on and off in Windows for several years. If I open up amdcccle, the overscan slider is at 0 (where it should be), but I'm seeing underscan. This changes if I drag the slider off 0 and back again, but only lasts until the next reboot.
Disabling underscan completely fixed the issue for me (as per http://nixnote.blogspot.co.uk/2012/06/amd-catalyst-fixing-underscan.html), but obviously won't for anyone who needs it, but isn't having their settings saved properly.
This changes if I drag the slider off 0 and back again, but only lasts until the next reboot.
I have same issue with Catalyst on my laptop and HTPC.