After waiting several weeks after Snow Leopard was released, I finally received my Snow Leopard Up-To-Date upgrade disk for my 13" MacBook Pro. I had already installed Snow Leopard on that machine so... shhh... I tried it on my 10.5.8 iMac instead. The install app wanted to upgrade 10.5.8 to 10.6, and provided no option to reformat and install. However, I was able to do that by rebooting off the install disk.
I then retried the Up-To-Date disk on the 10.6 iMac. The install app refused to work on 10.6. However, again, I was able to reformat and reinstall by rebooting off the install disk.
The Up-To-Date disk not only works to update 10.5 to 10.6, but also can do a clean install of 10.6 on any supported Mac.