I'm running into the same behavior, Kubuntu 9.04 with Catalyst 9.4 (final) amd64.
DPMS modes blank the screen but leave the backlight on.
I have tried with a Radeon 3850 and a 4850 with the same results. I've tried nearly every variation of an xorg.conf I can come up with with no results. vbetool is able to turn off the screen through dpms, but that isn't really a good solution.
Anyone running Catalyst 9.4 on another distro that has DPMS working? I'd like to know if this is a driver bug or an Ubuntu bug.