When it comes to Mac OS X, Apple has always had the luxury of not really having to care about piracy. Apple makes most of its money via hardware, and since Mac OS X only runs on Apple machines (technically), even if you do pirate your copy, you already gave Apple the big bucks when you bought your Mac.
The end result of this is that Apple never had to put any Genuine Advantage stuff or serial key mechanisms into its client operating systems. Windows, on the other hand, does have this stuff as an integral part of the operating system, since Microsoft doesn't have a hardware division to fall back on.
So, when it comes to the cheap Snow Leopard upgrade, there are no checks or safeguards built into the operating system, and you can buy one upgrade copy, and install it on as many machines as you want - whether they be Leopard or Tiger.
The EULA, however, does not allow you to. The EULA which comes with the Snow Leopard upgrade disk limits you to one machine, and only Leopard is eligible for an upgrade. So, this raises the question: how do you feel about this? Will you upgrade any remaining Tiger machines you have? Will you upgrade several machines using the same copy? Do you actually care about any EULA at all?