Have submitted this as a bug with redhat (http://bugzilla.redhat.com/show_bug.cgi?id=844022
) but thought I might mention it here too just in case someone else is having similar problems.
Did a fresh clean install of F17 on a 2009 Mac Pro using the install DVD, did all the updates, added all the other programs that the owner uses, noticed that the fans were starting to get noisy.
Installed gkrellm, all 8 processors are running constantly at around 33% capacity and temperatures are around 85 C. Open system monitor and looked at processes, Xorg is using one cpu at 0.5%, gnome-shell is using all 8 processors at 33%, everything else is at idle.
Did another clean install, this time just the standard graphical desktop install. No updates, only extra program added was gkrellm, all 8 processors are running at 33% and temperatures were in the 80's and climbing.
Reinstalled F16, no more problems. Owner can rip DVD's whilst listening to music and edit photos all at the same time, temperatures hanging around 37 to 39 degrees.