Jeff's blog post explains what's new in R2 Hyper-V.

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Kudos to the Hyper-V team for rolling out R2, and some of my favorites, like sconfig (http://blogs.technet.com/virtualization/archive/2009/07/07/windows-server-2008-r2-core-introducing-sconfig.aspx) SCONFIG and Live Migration: http://blogs.technet.com/virtualization/archive/2009/05/06/microsoft-hyper-v-server-2008-r2-release-candidate-free-live-migration-ha-anyone.aspx