I've seen a few articles about upgrading to Lion but most leave out the most important steps. Two steps that you should really do before any software update. In fact even when or if you need to update to the latest Snow Leopard (10.6.8) you should do these first.
Yes your Mac must have an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon processor to run Lion. Find out if your Mac has one of these processors by clicking the Apple icon at the top left of your screen, and choose About This Mac. Yes you need to have the latest Snow Leopard installed before you update. Since you purchase Lion with your Apple ID you are free to install it on all your machines using the same Apple ID. The Snow Leopard upgrade is another matter. It used to cost $129 but guess what? You can buy the DVD from the Apple Store now for $29.99. Finally you need to be near a network with enough bandwidth to download the Lion upgrade from the App Store. Apple will let you hang out at their retails stores for this purpose or you can just let it run overnight on your home network. If you will be doing multiple installs there is currently no way to save the trouble of downloading to each Mac in turn.
So what are the two steps other articles seem to be leaving out? They are simple and can be a little time consuming but very important. And both can be done using the Disk Utility application. Basically do a Repair Disk Permissions and a Verify Disk on the boot volume. You can do this after you have booted from your Snow Leopard install DVD if you are installing Snow Leopard. Everyone else will have to find their boot DVD (you have one right?) should the Verify Disk fail. Trust me, if your disk cannot pass muster on this there is a pretty good chance the Lion Install will fail and no-one wants that.