We have just released a prebuilt Windows 7 evaluation virtual hard disk for use with Hyper-V that you can download from here:

http://www.microsoft.com/downloads/details.aspx?FamilyID=606ae07e-b7db-405b-974b-dd61fc41add4

Windows validation is required to be able to download this – and the copy of Windows is limited to 90 days.  But it is still a neat and easy way to check out Windows 7 if you have a Hyper-V server handy.

Cheers,
Ben

http://blogs.msdn.com/virtual_pc_guy/archive/2009/10/13/windows-7-eval-vhd-now-available-for-hyper-v.aspx