This is a good time to "go legit" as a Mac user. Maybe you bought the singler-user install disc for Leopard and over the course of time it happens to have found its way onto more than one Mac in your house. Maybe something similar happened with iWork and/or iLife. Well, for $230 you can get a "Family Pack" of the Mac Box Set, which will give you the right to install Snow Leopard, iWork, and iLife on up to five computers that you own. Amazon had just recently dropped their price to $200 for the Mac Box Set Family Pack with Snow Leopard. I don't think there's ever been a less-expensive time to get "legit" with all your Apple installations. Every Mac user I know appreciates not having to jump through the hoop of entering in a "Product Key" when installing OS X or iLife/iWork from a DVD. Supporting Apple by buying licenses for all your computers is doing your part to encourage Apple not to go down the path of product activation (although I can't imagine Apple foisting anything like the so-called "Windows Genuine Advantage" on its users).
While you wait for the hours to pass by, take a break from refreshing the tracking page for your order and make sure that you have a current, working (please note the key word "working") backup of your current installation. If you haven't already, this would be a good time to run CarbonCopyCloner or SuperDuper. A fully bootable backup is your best insurance policy against anything bad that might happen... but only if it works! After you update your backup, reboot your Mac while holding down the alt/option key, and boot your Mac from your backup drive. (Read more from Apple about booting from alternate drives.)
Did it work? Great! Unmount, eject, and physically disconnect the drive and set it aside.
Did it fail? Oops! Well, better to find out sooner rather than later. This would be a good time to figure out why.
Pro Tip: If you have an Airport Extreme with a USB drive attached or other external hard drive, download the latest versions of your favorite/most important apps. You can hook this to your Mac(s) after upgrading them and quickly update as needed. (This is probably more important if you plan to use "Erase and Install" but it's not a bad idea in any case.)
It's a good idea to keep an eye on the various Apple-related websites [ahem] and discussion forums to see if there's any widespread reports of problems. I know I can't probably convince any of you to wait until Saturday or Monday to see if everything goes smoothly (and I have no doubt that I'll be jumping in myself), but at least while you're waiting for the FedEx guy to arrive, keep a proverbial ear to the ground to see how all those people who rushed to the Apple retail stores are doing.
A lot has been made about the proper way to upgrade a Mac, and I won't try to convince you to do or not do anything in particular. One of Snow Leopard's new features is a new, streamlined installation process, where you don't need to boot from the DVD manually to install -- the installer tool takes care of all those steps for you.
My preferred process is a bit more hands-on: first I shut down my iMac (after putting the new DVD in!) and disconnect everything from it except the keyboard and mouse. I call this "minimizing the variables." Then I reboot and hold down the alt/option key and choose the installation DVD (you probably don't have to hold down the alt/option key but it shouldn't hurt anything). Then I go through the options very carefully. When I am installing a totally new version of the OS, I do an Erase and Install. Would Archive and Install work? Almost certainly, but I take it as a good time to do some "spring cleaning." Then I sit back and wait.
While the iMac is upgrading, I run SuperDuper on my MacBook (which is my work computer) so I know I have a very recent backup of it. If the iMac upgrade goes smoothly and it's working with my key applications, I'll upgrade the MacBook in the same way. At the very least I know that I have one working Mac, and my SuperDuper backup of my MacBook and iMac are the safety net beneath my wings.
It's an exciting time to be a Mac user, and I'm looking forward to Snow Leopard too, but a few careful steps of preparation today (did I mention to verify that you can boot from your backup drive?) can save a lot of wailing and gnashing of teeth tomorrow.