According to MacRumors, Amazon is now taking pre-orders for Mac OS X Snow Leopard.
Amazon has also issued an “upgrade path alert” warning customers that Snow Leopard is only available for users running Leopard on an Intel machine:
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.
In addition, Amazon is also taking pre-orders for OS X Snow Leopard family packs, and Snow Leopard Server.