Likely Mac Apps will have similar Terms that exist for the iTunes Apps, which an be installed on up to 5 devices. With this limitation, somebody would not be able to buy iLife '11 and give it out to 30 friends. Might actually result in more profit for the developer.
That's a popular myth but it's not true. You can install iPhone apps on as many devices as you have associated with your Apple ID. There isn't a limit of 5.
I believe it to be the same for the Mac app store: you can put the app on as many Macs as you want.