To add my 2 pence to Gkeerthy's comments, if your storage supports VAAI and UNMAP, there is a great article from Cormac Hogan showing it in action here: http://blogs.vmware.com/vsphere/2012/04/vaai-thin-provisioning-block-reclaimunmap-in-action.html
This should at least put you on the right track I hope.
Thanks for sharing your results if you get somewhere, as I will probably be looking into this too at some point soon 