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