You can ALREADY do this. This is a feature they've already put out there if you have the most recent version of iOS 4.
Go to SETTINGS > STORE and check that it's logged in correctly. Give it a second, the system has to enable that feature while you're on that screen. Then go to the 'updates' section of the app store on the phone and you'll see a 'purchased' bar on the top. Click that to see everything you've ever bought, ready to re-download.