WindowBlinds 6.0 & XP x64 Support
The most obvious reason I would think it is supported is that your own page for WindowBlinds 6 states unequivocally:
*Start Quote*
Windows XP (32-bit or 64-bit) or Windows Vista (32-bit or 64-bit) are required for WindowBlinds 6 (users of Windows 2000, 98, and ME can purchase WindowBlinds Classic). No additional requirements are needed to use it.
*End Quote*
I have then a simple question of, when will it be supported? If the answer is "Never," I think I may need to speak with customer service.

I stopped using 2003 almost a year ago and WB6 is now working fine under Vista x64, so it's all good.