I've been waiting for quite a long to post this information. Hyper-V now supports systems with up to 64 cores (we call them LPs). This is 4x the number supported in the original release just 9 months ago.

The devs on my team write the code and did the tuning to make this possible. It was our biggest stretch goal, and we nailed it. There was a substantial amount of work that went into it. Not the least was wrestling with machines that support that many cores. They live in racks, draw enormous armounts of power, require enormous amonts of storage, can take hours to boot, and are very finicky about any changes to configuration. They're also quite expensive, especially when considering thre crazy amounts of memory they need to do use work.

It was an awesome sight, working with a machine running 384 VMs using 64 procersss and 256 GB of RAM. The 384 VM value is a tested, supported limit, and not an architectural limit, so you can read between the lines there.

 He'res the official link for more info:

 http://blogs.technet.com/virtualization/archive/2009/05/12/hyper-v-in-ws08-r2-release-candidate-bringing-more-to-the-table.aspx