Over at my personal blog (www.mikepoulson.com) I have a post on using Differencing Disks with an SCVMM template. 

The main issue I ran into when first doing Diff disks with VMM was making sure the Parent VHD was on the target HyperV host.  The PowerShell script at http://www.mikepoulson.com/2011/07/how-to-create-differencing-disk-vm-from.html does that work for you.