One of the most exciting new virtualization features in Windows Server 2008 R2 (R2) is clearly Live migration. With Live migration you can move running VMs from one Hyper-V physical host to another without any interruption of service. Live migration is part of Windows Server 2008® R2 Hyper-V and Microsoft® Hyper-V Server 2008 R2.

R2 and Hyper-V server R2 are expected to ship early in 2010, the betas are currently available.

Resources:

Information about Windows Server 2008 R2
Download the beta of R2
Download the beta of Hyper-V server R2
Architecture Overview of Live migration
Read a Step-by-Step guide on how to use Live migration

Video demo on TechNet Edge
Video demo on YouTube