Check out a quick video giving a demo of Thin Provisioning in vSphere 4.0.

<embed src="http://blogs.egroup.jasonmangone.com/wp-content/uploads/2009/05/thinprovisioning.wmv" autostart= 0 width="500" height="300" align="center" style="border: 2px solid #808080; " > <noembed>A Media Player is required.</noembed></embed>