It looks like at least one online vendor has started selling Windows XP Professional x64 Edition in OEM format.  The price is $142, but you have buy a piece of hardware with it to make it "legit."  They even have a $1.99 keyboard as an option!

Notes: (1) You have three ways to qualify to purchase this OEM product: (a) Buy them in multiple of three. (b) Buy it with any hardware. Yes, ANY hardware. You can be creative here. We have added a few low-cost hardware as optional items to make it easy for you to qualify.