As with iOS apps, desktop apps may be re-downloaded at any time; that essentially means that Apple's backing up all your store purchases for you. Run the Mac App Store app on any Mac linked to your iTunes account and click the Purchased tab, and you can quickly re-download any app you already own.

One key element of current Mac App Store confusion centers on what insurance companies might call pre-existing conditions. If you purchase an app directly from the developer, can you upgrade it from the Mac App Store, and enjoy its other benefits (like the right to freely re-use that software on all your Macs)? In a word: .

Some developers are offering customers . If that's something that interests you, your best bet is to contact developers directly for information on their policies.