I was about to post my problems with the grandr applet on fc10 x86_64, RV350/9550 (using the free radeon driver), properly detecting and configuring a second display when a search turned up your post.
I haven't been able to get dual displays on my rig from a linux distro for quite some time, but discovered it *works* using the grandr applet out of the box in Jaunty (same pc and driver). Jaunty+grandr correctly identified the available resolutions of the second monitor, informed me it needed to set the virtual display (in xorg.conf) and did so. However, it did take a couple of x-server restarts and a reboot to get it running properly. Note however it will *not* boot into the desired dual monitor config, and I have to invoke it from grandr every time (which is ok, because I seldom need to run dual displays).
Feeling brave, I decided to then try it in my FC10 install, thinking it is running more or less the same, up-to-date X-server components. Unfortunately though, in fc10, grandr does not correctly detect the second display, or respond correctly to any setting I apply to it. It reset both displays to 800x600, when I want to run the left at 1280x1024 and the right at 1024x768. When I tried to fiddle with the position in grandr (left,right) it restarted x-server into a black screen on both displays.
I copied over xorg.conf from jaunty to set up the correct virtual display resolution, but (although x-server restarted ok with the 'foreign' xorg.conf), grandr isn't behaving any better.
Will I need to resort to more drastic xorg.conf ministrations in fc10 to get this working? Any reason to suppose it will work, before I waste hours on a hopeless cause?
Anyhoo, your report of cli randr commands working is at least encouraging, so perhaps I'll give that a shot first.