Some time ago VMware together with Intel and AMD announced that they were taking steps towards enabling cross-platform vmotions. The intention was to move a vm from one host (intel) to another host (amd) with vmotion technology. The companies now announced they dismiss their plans towards cross-migration plans.
The reason why this effort is currently on hold is the way the migration becomes available. The idea is to hide all instructions from both processor types that differ. In that way you have two "equal" processors/instruction sets. Because the instructions that differ between the two architectures are growing in numbers these instructions are more and more being used. Some of these instruction are only detected by initialization (start/boot), if you then migrate a vm to another architecture the OS might still think that certain instructions are available and will do calls to these instructions. If this happens the VM becomes unstable and might crash depending on the instructions used. In terms of High available or fault tolerance (ESX4) this is something you don’t want.
AMD however said that it nevertheless will put a lot of effort in making it possible in the future, not surprisingly because they want to regain market share they lost with the delayed AMD quad core introduction.
Hopefully both parties do agree on developing some sort of solution for this, because the demands from the market are still there.