What if I have multiple Macs? Can I install it on each one?
Upgrading to Lion? You’ll only be able to buy it through the Mac App Store, Apple says.
This is one of the biggest benefits of Mac App Store distribution: As with any applications you purchase from the Mac App Store, you’ll be able to install Lion on any Macs that are authorized with the Apple ID you used to purchase the OS. Which means that if your family has four, five, six, or more Macs, a single $30 payment will let you install Lion on every machine.