Walt Mossberg has answered a Snow Leopard upgrade question that has been on just about everyone's mind.
Do you need the full $169 box set if you are upgrading from Tiger? The answer is apparently no, not exactly, although that is what's required by Apple's EULA. You will be able to install the $29 individual upgrade or $49 family upgrade on any Intel Mac regardless of whether it's already running Leopard. According to Uncle Walt, as posted on the All Things Digital site:
"Apple concedes that the $29 Snow Leopard upgrade will work properly on these Tiger-equipped Macs, so you can save the extra $140."
What's not 100% clear from this report is whether the Snow Leopard install would work as an upgrade, or only as a clean install (on a newly formatted drive), as Lifehacker suggests. Since some experienced Mac users prefer to do a clean install with every major OS upgrade -- either reinstalling apps and files from backup, or using Migration Assistant to pull over from the old configuration -- this may not be a drawback for everyone. Wired's preview of Snow Leopard (based on a pre-release version of the OS) suggests that they were able to do an upgrade install from 10.4 straight to 10.6 using the conventional SL disc, but your mileage may vary.
So there you have it. If you have Leopard running on your Intel machine, you will be fine with the $29 single or $49 family versions on sale Friday morning. If you're willing to wipe down your Tiger install and start fresh, the $29 installer will probably work for you too -- but you'll be in violation of Apple's licensing agreement, making you an OS pirate. Just so you know.