Just thought I'd follow up on my post from around august.... it seems that the latest kernel has indeed fixed the cool 'n quiet issues with my Athlon X2 3800. basically the following messages were appearing;
powernow-k8: ignoring illegal change in lo freq table-2 to 0x2 powernow-k8: transition frequency failed
and both cores were going at full 2.0ghz speed.
Now with 2.6.9-42.0.3.ELsmp the errors have disappeared and both cores are running at 1ghz when the system is idle.
I'm very surprised as I checked the changelog (rpm -q --changelog) and there was no specific mention of any fix that I could find.
I had actually toyed with the idea of running another distro on the server as it lives in my garage which, obviously, has no Aircon and the summers here in Brisbane get pretty hot.