Versions of Microsoft Windows for AMD64 and EM64T are now available:

Windows Server 2003: http://www.microsoft.com/windowsserver2003/evaluation/trial/default.mspx

MSDN Subscriber Downloads have both versions available now, under Windows Server 2003 and under Windows XP Professional (not the SP2 branch) respectively.

X64 versions are built from the same code base as the 32-bit Windows Server 2003 SP1.