We are always keen to test out the latest features provided by manufacturers, so we've implemented VMware Storage I/O control on our own VMware 4.1 environment. We seen performance improvements on multiple storage processes, i.e. running simultaneous storage vMotions on Virtual Machines did not mean other storage I/O process slowed down, so end users were not impacted by IT back-office maintenance tasks. Thus we could carry out heavy storage I/O tasks during the day where normally we would have scheduled these processes out of hours. Storage I/O Control is only available on vSphere 4.1, and only VMFS 3.46, so you'll have to upgrade all your Hosts & reformat your VMFS Datastores.

More from VMware on Storage I/O Control here
http://www.vmware.com/products/storage-io-control/