Here are some news. I now installed 10.5.6 on my A8N-E FM/S board which is equipped with an Athlon 64 4000+, not with an Opteron. After installing your new kernal and the VoodooPower package I started the Generic Power Management Tool, but it looked the same like on the Opteron system.

After this I looked in the System Profiler which Extensios were loeded. There was NullCPUPowerManagement.kext loaded. I deleted it for testing and rebooted. Result: Kernal Panic because of VoodooPower.kext. The same happend on my Opteron system, photos are in the attachment.

I now tried to add AppleACPIThermal.kext, first to the Plugins folder inside AppleACPIPlatform.kext, then directly in extensions, but it isn't loaded in any case. I also tried blacklist=0, after this AppleHWSensor.kext is loaded on my Athlon system. The Opteron system crashes, it does a direct reboot whitout a kernel panic.

EDIT: During boot and in the Console I get the message 'cannot resolve dependencies for kernel externsion /.../AppleThermal.kext'

Could you take a look why AppleACPIThermal.kext isn't loaded in 10.5.6 anymore? Some more people have problems with hot AMD cpus, maybe this is the key. I added my complete 10.5.3 AppleACPIPlatform.kext to download here. There are some more plugins inside it.

I know why AppleACPIThermal.kext doesn't load...
Look in the Info.plist... the dependencies on the bottom have incompatible versions set...
Look through the Info.plists of the other kexts (the dependencies) and fill in the same version as the compatible bundle version...
what you can try for enabling VoodooPower is to use boot argument -alt-get-frequency...
It directly gets the frequency without a multiply divide operation (which on some AMD's causes a crash).
If you look at the kernel panics on top you will see divide error...
Try it and let me know...