SQL Server 2008 R2 fully leverages new Live Migration feature delivered as part of Windows Server 2008 R2 Hyper-V release. This provides even greater benefits to virtualizing & running SQL Server instance in a virtual machine by providing zero planned downtime. You can also leverage SCVMM Performance and Resource Optimization to dynamically load balance your SQL Server workloads in a multi-machine/multi-instance environment. A great video explaining this is more detail:
“ we ran a test simulating ‘consolidation of multiple applications’ onto a new 32 core server running Server 2008 R2 and SQL Server 2008 R2. We used three of the most common consolidation methods: virtualization (one machine with multiple VM’s each with one SQL Server instance), instance (one machine with multiple SQL Server instances running natively), and database (one machine with one SQL Server instance and multiple independent databases). As this chart shows, we were able to achieve consistent results across each consolidation method even up to 40 apps running simultaneously (and yes, this does mean 40 VM’s running simultaneously)!