Great news at http://blogs.msdn.com/ie/archive/2006/11/30/ie6-and-ie7-running-on-a-single-machine.aspx

There is now a Virtual PC image available for XP SP2 with IE6 for free so you can run IE7 and IE6 on the same machine without any need to buy an additional Windows license.

Fabulous!

Thanks
-Dave