Automatic app downloads are being integrated into OS X Mountain Lion, checks show. The feature is present in the latest Developer Preview of the OS; under the new option, titles bought at the Mac App Store should, in theory, appear on any other Mac signed in with the same Apple ID. In practice though, the store is marking titles as "installed" on secondary Macs without actually copying any files. The function will likely be turned on in future Developer Previews, or at least the final version of Mountain Lion, due sometime this summer.
