Amazon is offering Snow Leopard at $29 and is slated for a September release as announced by Apple earlier this year at their World Wide Developers Conference.
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.
Apple is also offering Snow Leopard upgrades for $9.95 for those who purchase a qualifying computer or Xserve between June 8 and December 26, 2009, that does not include Mac OS X Snow Leopard from Apple or an Apple Authorized Reseller located in the United States or Canada, or the Apple Online store.