Despite Apple's suggestion on the Snow Leopard specs page that, when going from Tiger 10.4.x to Snow Leopard, you should buy the box set...
Upgrading from Mac OS X v10.4 Tiger: If your Intel-based Mac is running Mac OS X v10.4 Tiger, purchase the Mac Box Set, which is a single, affordable package that includes Mac OS X v10.6 Snow Leopard.
[robg adds: Jason Snell covered this interesting fact in his Snow Leopard review, where he pointed out a key benefit of Apple's honor system approach:
...the standard version of Snow Leopard is a bootable "full install" disc that doesn't actually check for the presence of Leopard in order to install. This also means that if, at a later time, you want to wipe your hard drive and reinstall Snow Leopard, you won't have to first install Leopard and then run a separate Snow Leopard upgrade on top of it. (That sound you hear is a thousand IT managers sighing with relief.)There are advantages to the box set, in addition to complying with Apple's licensing terms -- you get new versions of iLife ($79) and iWork ($79), which essentially lowers the cost of Snow Leopard to $11.]

