If you mount the Leopard/Snow Leopard DVD in Windows 7 x64 and try to launch the setup program to install all drivers on an unsupported system you will get an error, yes.
But, if you explore the contents of the DVD and work your way down to the folder X:\BootCamp\Drivers\Apple (where X: is the letter of your DVD) you will find a file called BootCamp64.msi
Double click on this file and BootCamp and all x64 drivers will be installed without issues.
This is how I have successfully installed Windows 7 x64 on my uMB and Mac Mini.