Apple officially states that Tiger users must buy a Snow Leopard box set to update; however, that may not be the case.
The cheapest Snow Leopard box set costs $169.99 and though it includes iLife and iWork its no where near as cheap as the $29 cost of Snow Leopard alone.
According to Walt Mossberg, Tiger users may not need to shell out the extra $140.
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 heres 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.
The snow leopard one user dvd for installation does actually update from the latest version of mac os x 10.4 tiger to mac os x 10.6 snow leopard without any problem.
To update from tiger I recomend you start the installation at startup from the disk by pressing alt(option key) and then booting from the snow leopard install dvd but not erasing the disk just upgrading the OS.
Yes it is true, I have Snow Leopard, and I installed it on an empty hard drive, so I'm not sure if you can just upgrade from Tiger, but you sure can make a backup, erase your hard drive, install Snow Leopard and restore.