Quoting from the AMD official announcement:
…AMD today disclosed details about “AMD-V Extended Migration”, a feature dating back to the First-Generation AMD Opteron processor that enables virtual machines to migrate between different versions of AMD processors. AMD has worked with VMware, the industry leading virtualization software provider, to refine this feature in order to enable a key functionality designed to help IT managers fully leverage the benefits of virtualized IT infrastructures. In addition, to further contribute to the industry discussion around how to implement and manage virtualization capabilities…
AMD-V Extended Migration provides the necessary support for virtualization software to mask the differences between CPU generations, facilitating the safe live migration of virtual machines between servers running different generations of AMD processors. This includes existing single-core and dual-core processors and all future AMD processor revisions, including the upcoming “Barcelona” Quad-Core AMD Opteron processors, the first native x86 quad-core microprocessor.
Novell, Qumranet, Red Hat, Sun Microsystems, Virtual Iron, VMware and XenSource are among the leading virtualization software vendors with the capacity to take advantage of AMD-V Extended Migration…
AMD released a very technical 17-pages whitepaper about this technology which worths a read: Live Virtual Machine Migration on AMD Processors.
Intel is working on a similar tecnology called Flex which will appear on upcoming quad-core motherboards.