Apple has released iTunes 10.3 (http://www.apple.com/itunes/) for Mac OS X and Windows. It introduces support for iTunes in the Cloud, which automatically pushes purchased apps, books and music to all your devices signed in with your Apple ID.
You can browse past iTunes Store purchases and re-download them to you machine or iOS device, if the files aren’t already present. You can also enable automatic auto-downloading across all devices, so that any song, app, or book you buy on your iOS device will download to your Mac, as well.