Amazon now have Mac OS X 10.6 Snow Leopard available to pre-order.
Amazon do say
Please note, that only Apple OS X Leopard users are eligible for the Snow Leopard upgrade. Tiger & earlier OS users will need to purchase either versions of the upgraded Mac Box Set. Also, Snow Leopard will only run on intel-based Mac computers.
Oh, if you want to pre-order Snow Leopard Server, you can do that too.