There was another thread on this a long time ago, but I think the conclusion was incorrect, so I am starting another one. It appears that there is a bug in VMware Server, at least up through 1.0.7, which causes 64-bit guests to not be supported on processors that could actually support them just fine.
I am running on a Phenom 9850 Black Edition (B3 rev) processor, under VMware Server 1.0.7 for Linux. When I try to create a 64-bit machine, VMware gives me the error message "64-bit guest operating systems are not supported by this host and will not run". This, of course, is not true. While I obviously can't speak for AMD, I have read that AMD has stated that all Phenom processors, without exception, support 64-bit guests. I could ignore this incorrect warning message, except for the fact that VMware Server won't start the VMs automatically if they have this error message, which means I can't deploy 64-bit VMs on a system that can obviously run them just fine. They will start and run perfectly well if I start them manually.
I believe there are two things causing the confusion: First, the TLB errata on the earlier AMD Phenom's, which is a bug which can theoretically cause a system lockup when you're using virtualization but which doesn't, as far as I've seen, directly prevent the system from being capable of running 64-bit guests. Secondly, there were some improvements to the virtualization support in the Phenoms, which perhaps is part of the reason VMware Server doesn't recognize them as supporting 64-bit guests (even though they actually do).
I do have 64-bit guest support enabled in my BIOS, and I have verified by looking at my processor flags that the 64-bit guest acceleration support is enabled in my processor and BIOS. This definitely appears to be a VMware Server bug, causing it to not recognize the AMD Phenom processors (at least the newer B3 revs).
So basically, to sum up, VMware Server, at least through 1.0.7 on Linux (and maybe other platforms) doesn't seem to properly detect the AMD Phenom processor and allow 64-bit guests as it should. I would suggest to VMware that perhaps a good idea would be to allow an "advanced" setting to override this detection, and just allow the admin to force the 64-bit guests to run anyhow (as I was doing with VMware Server 1.0.6, except that it wouldn't start the VMs automatically because they weren't "supported"). They should also be able to start automatically, as 32-bit VMs and 64-bit VMs on processors recognized properly by VMware Server do. The failure of VMware Server to recognize the Phenom processors (at least the B3 rev) certainly looks like a bug, in any case.
I'm willing to provide more information if anyone at VMware wants to fix this. Just let me know, and I'll be glad to help.