I recently picked up an old PE2970 running 2x Opteron 2200's to use as a VM host to tool around on as a lab box. Threw ESXi 5.5 on it, and during the install it threw a single error about hardware_virtualization being disable. Checked BIOS, found relevant option: enabled. Try again. Same problem. Okay, time to upgrade BIOS. Upgrade to current (4.2.1). Same issue. I didn't see any warnings in the hardware compatibility list, and the relevant BIOS option is on, so now I'm stumped. What the heck is up here? Any fixes? I'd like to have a couple 64 bit guests to toy with, and it's looking like this would throw the brakes on that pretty quick.
So the physical CPU supports AMD-V? And you are sure it is enabled?
If you have enabled AMD-V could you try the VMware CPU identification utility to check whether AMD-V is enabled or not.
http://www.vmware.com/download/shared_utilities.html
I'm running a 2970 with ESXi just fine, and have been for several years. It's got 2 Opteron 2431s in it.