Apple has previously stated that while Leopard users simply need to purchase the $29 DVD to upgrade their machines to Snow Leopard, Tiger users with Intel machines needed the $169 Mac Box Set to upgrade. As a bonus, those users get iWork ’09 and iLife ’09 as well.
It seems that the $29 Snow Leopard DVD can indeed update users still running Tiger, however.
Walt Mossberg of the Wall Street Journal:
And, for owners of Intel-based Macs who are still using the older Tiger version of the Mac OS, Apple is officially making Snow Leopard available only in a “boxed set” that includes other software and costs $169. The reasoning is that these folks never paid the $129 back in 2007 to upgrade to Leopard. But here’s a tip: Apple concedes that the $29 Snow Leopard upgrade will work properly on these Tiger-equipped Macs, so you can save the extra $140.
Lifehacker mentioned this as well in their “Prep your Mac for Snow Leopard” piece.