Mike DiPetrillo, Specialist System Engineer of Industry Research and Competitive Analysis department at VMware, describes on his personal blog some of the technical issues when virtual machines are live migrated between Intel and AMD CPUs:
...Basically you're running an OS on an Intel box and let's say the processor supports the SSE3 instruction set and your app happens to use that instruction. Now you migrate that to an AMD box that doesn't support SSE3 but the app is still using it and trying to use it. BAM! Your app and your OS will crash. This can happen with VMotion and Microsoft Quick Migration. Actually anyone that does live migration will get impacted by this. There are several "user mode" instructions like this that we can't mask out at the virtualization layer...
But DiPetrillo goes further and states that the Microsoft and Citrix implementations of this technology, included in upcoming Hyper-V and the current XenServer, may be a serious risk for virtual machines:
...With the Xen based live migration and Microsoft Quick Migration they do not perform the check and so you can actually do the migration but your app and your OS may die as a result.
...that's why we say you can't migrate from Intel to AMD just yet and this is why anyone that says they can do it is lying to you or just don't understand the technology. The later happens to be true with most of VMware's competitors - especially the field sales...