There's stuff all over the Windows 7 training materials about .VHDs. The Windows 7/Server 2008 R2 installation process uses .VHD in place of .WIM files and the Windows Deployment Service in Server 2008 R2 can deploy .VHDs as readily as .WIM files for supported OSes.
That's only a big deal if you're planning to do deployments that way, but it's nice that they collapsed a whole bunch of functionality into a single format. It borders on elegant, which is pretty damned remarkable coming from Microsoft.