Here's what I found:
On the page http://www.microsoft.com/windowsserver2008/en/us/licensing-faq.aspx
at the bottom it says:
Q. Do I get downgrade rights if I receive my server license with the purchase of new hardware through an OEM or if I acquire my software through a retail store? A. Yes. Downgrade rights are applicable regardless of which channel you used to acquire your license.
Q. I understand that I get downgrade rights, but where do I go to get the other media and product key? A. If you purchased licenses via Volume Licensing, you can obtain media and keys via the Volume Licensing Service Center. For OEM and FPP customers, you can order a fulfillment kit via the Windows Server Fulfillment Site. For more information, please click here.
Clicking `here' takes you to:
In the 5th paragraph, under Windows Server 2008 R2 Downgrade Licensing Rights, it says:
The following table shows the available downgrade options for each edition of Windows Server 2008 R2 the customer is licensed to use. If a customer already has media and product activation keys for the earlier version of the product for which the customer already has downgrade rights, the customer can use that media and keys to downgrade to an earlier version. If the customer does not have media and keys, please see below on how to obtain a fulfillment kit only and which versions and editions are available for order.
Later on that page under "Original Equipment Manufacturers (OEM) Customers", then under "Obtaining Product Keys and Media for Previous Versions", it says:
1. From a previous license. Customers may use media and/or a product key from a previous license in order to install, activate and run an earlier version of the product. The product key and media must match. If the product key being used has already been activated, the customer may not be able to activate over the internet. In these cases, the customer will be prompted to call the Activation Support Line and explain their circumstances to the Customer Service Representative. Once it is determined that the end user has a valid Windows Server license the Customer Service Representative will help them activate their software. For more information on Product Activation Call Centers go to http://www.microsoft.com/licensing/resources/vol/numbers.mspx.
Some OEMs have pre-activated their software. In these cases, a customer would not need to activate over the internet or via the phone.
Option 2 is that my OEM may provide `downgrade Windows Server 2003 R2 media' and then only `only if their customer requests it'.
Option 3 eventually takes one to a Microsoft site which lets you order a Windows Server 2003 R2 CD for $20.00.
So, bottom line, the downgrade is do-able, but it wasn't an easy path to find out how to do it. Maybe by telling you we can prevent some other poor soul from having to dig around for the information...