Mike Kolitz has just released "WIM2VHD".  This is a tool that allows you to take the WIM file off of a Windows 7 or Windows Server 2008 R2 install DVD and turn it directly into a virtual hard disk that can be used by Hyper-V, Virtual Server or Virtual PC.  Not only is this handy - it is *fast*.  Usually under 10 minutes to get up and running.

You can download the tool from here: http://code.msdn.microsoft.com/wim2vhd

And you can read more about it (and see a video guide) on Mikes blog here: http://blogs.msdn.com/mikekol/archive/2009/02/06/announcing-the-windows-image-to-virtual-hard-disk-converter-wim2vhd.aspx