The next version of Microsoft Virtual Server will NOT support 64 bit Virtual Machines, so even if your run Virtual Server x64 edition on 64bit hardware, you can't run 64 bit guests.

At IT Forum last November, when I asked about this, it was still unclear. In December I got contractive reports on the issue, but last week Microsoft confirmed to me, that current plans do not include 64 bit guest support.

This is a disappointment and keeps Vmware a step ahead, still.

v Next will support the new Intel VT and AMD Pacific CPU's, which make use of a new ring in the CPU architecture. This ring will boost performance because difficult tasks in software vitalization will be done by the new CPU's.

Maybe Vmware latest move, (making its next Virtual Server product free and with 64 bit guest support), will make Microsoft reconsider it's decision.
Virtual Server v Next will be released later this year, somewhere in the 2nd half I guess.

The Windows Hyper visor, virtual server release in the Longhorn server timeframe will support 64 bit guests, but that is far in 2007.