As posted on VMwareInfo.com:
When crafting VMware solutions for clients, I always stress the importance of buying HOST servers in at least pairs. This guarantees that every server will have a vMotion partner in the farm. When clients buy servers that are not vMotion compatible with another server, the VMs they place on it are at risk. Since vMotion and DRS are relying on compatible processors to function, I routinely see trapped Virtual Machines running on ESX software that falls behind in the patch cycles due to lack of a maintenance window. The DRS pairs, on the other hand, are upgraded more regularly since no downtime is incurred on the Virtual Machines.
Here are some links to some matrixes provided from hardware vendors to try to mix and match compatibility but they aren’t updated nearly enough.
For additional information, see the following Articles –
VMotion CPU Compatibility Requirements for AMD Processors
VMotion CPU Compatibility Requirements for Intel Processors