From BillG's keynote today...

There was a Windows Hypervisor demo, which showed a running Windows 2003 Server image imported from Virtual Server R2 (which emphases the transportability of Windows Virtualisation today and going forward). 
Virtual Machines were shown running Redhat and Windows 64bit guests, previously only 32bit guests were possible(x64 hosts has been available for a while).  Also a Windows Server guest was shown using 4 CPU’s, and 8 CPU support for guests was mentioned, in addition each guest can support up to 32GB of RAM, up from 3.6GB.
Hot add of a Network card and Memory was shown, which is a unique feature for Windows systems on a Virtualised platform.  The demo showed a Virtual machine with no Network card, a NIC was then added via the management tool MMC, and appeared within the running Windows Server guest, memory was also added and the machine which was originally using 4GB of Ram, had an additional 1GB added dynamically.