Except that it not supported in a clustered environment by anyone. Even RHEL lists it as a tech preview. Not to say that each of those items isn't great, but as someone who has spent the last month implementing a RHEL VM host cluster, KVM just ain't ready for primetime. Xen while it really does have it's bad points, does work great though.
Actually, just about any server now days has power management turned on by default, (and in the case of HP's c class blades almost impossible to turn off). If they reduce the load by 30%, assuming that they were running near peak capacity I could easily see them cutting their power bill by 25%, if not more due to cooling expenses.
Just wanted to point out, HP switches are not rebadged, the procurves are actually made by HP. They don't quite have as many bells and whistles as the Cisco ones, but they are heck of a lot cheaper.
Why would you cluster local disks? If one dies you lose the data on the disk, defeating the point of a cluster. If you want to share the data between the two, there are lots of different types of software that will keep it in sync, however since it's a pretty stupid idea it's not built into any of the standard clusters management tools.