An interesting article by Drew Robb on Enterprise Storage Forum examines the evolving relationship between virtual machines and storage.
One of the key questions he explores is whether the practice of over-provisioning storage for virtual machines actually diminishes utilization rather than enhances it, as some vendors claim. In some instances, virtual machines are provisioned with more physical storage than is available, causing enterprises to continually add storage to avoid conflicts, particularly if the VMs are equipped with snapshots and other auto-backup features.
Management issues begin to get more complicated once virtual servers start to rely on virtual storage. Even simple tasks like measuring CPU utilization can break down when confronted with radically different virtualization abstraction layers.
The article is rather long, with plenty of meat for all you techies, and at the very least points out that management of virtual systems will be crucial if virtualization is to gain a prominent place in the IT hierarchy.