When you purchase Lion through Snow Leopard’s Mac App store, the installer is downloaded to your Applications folder (Install Mac OS X Lion). After upgrading to Lion, this installer is deleted.
If you make a copy of this installer prior to upgrading to Lion, you can use it to install Lion on other Macs. If you’ve already installed Lion and didn’t copy the installer, you can re-download it from the Mac App store by holding down the option key while pressing Purchases.
The Lion installer is a package, and if you open it and navigate to /Contents/SharedSupport you will find an item named InstallESD.dmg. This is a disk image of the Lion installer, which can be restored to an external USB/FireWire volume, or burned to a DVD disc.
Although Lion requires Snow Leopard, this “requirement” is mostly based on the fact that the Mac App store is only available in Snow Leopard, and Lion currently is only available via the Mac App store. Using the above tricks will however allow you to install Lion from scratch, or on a supported Mac running Leopard. One caveat is that you cannot use Apple’s Migration Assistant if upgrading directly from Leopard to Lion… this requires the Migration Assistant Update for Mac OS X Lion 1.0 released by Apple on 7/19/11, which can only be installed on Macs running 10.6.8.
Although these tricks will give you the ability to install Lion on Macs you didn’t purchase Lion on through the Mac App Store, make sure you are doing this legally! If in doubt, refer to this Apple software license agreement document for Lion… http://images.apple.com/legal/sla/docs/macosx107.pdf