The information about enterprise licensing rules on Windows 7 for organizations has been released by a longtime Gartner anaylys, Michael Silver.
According to Silver, “…businesses that buy PCs before April 23, 2010, with Windows 7 pre-installed can downgrade them to Windows XP, then later upgrade them to Windows 7 when they’re ready to migrate their users. But PCs bought on or after April 23 can only be downgraded to Vista.”
Microsoft has not confirmed whether this information is accurate.
Source: Channel Register

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Lee
Under any current Microsoft OEM, Open, Enterprise or Select Agreement the EULA states that if you purchase the latest version of the OS you have the right to downgrade it to ANY previous version of the OS as you wish. The one caveat to that is that Microsoft will not provide you with support nor media to utilize your downgrade rights. This has always been Microsofts philosphy so I can’t see them stepping away from it. Please see the below link for further clarification right from the Microsoft website.
Google HTML Version:
http://74.125.155.132/search?q=cache:QhRPqUZGi9kJ:download.microsoft.com/download/d/2/3/d23b9533-169d-4996-b198-7b9d3fe15611/downgrade_chart.doc+Microsoft+Open+License+Downgrade+Rights&cd=1&hl=en&ct=clnk&gl=us
Original Microsoft Word File:
http://download.microsoft.com/download/d/2/3/d23b9533-169d-4996-b198-7b9d3fe15611/downgrade_chart.doc