What did bill realise he was the richest guy on the planet and could be nice to his customers or is he just lookin for beta testers to iron out bugs <i think this is why>
Windows XP Professional and Windows XP Professional Reduced Media Edition are eligible for the license exchange.
http://www.microsoft.com/windowsxp/64bit/upgrade/default.mspx
http://arstechnica.com/journals/microsoft.ars/2005/5/26/405
In order to be eligible to receive Windows XP Professional x64 Edition, your computer must have been ordered or built between March 31, 2003 and July 31, 2005 and must contian an x64 processor. You must also have a Retail (Full Packaged Product), System Builder, or OEM version of Microsoft Windows® XP Pro (32-bit) to exchange.
http://www.microsoft.com/windowsxp/64bit/upgrade/default.mspx
http://arstechnica.com/journals/microsoft.ars/2005/5/26/405