They don't show up in Purchases, but if you see them in the store, they are marked as "Installed" instead of giving me a button to buy them. From what others have figured out, the App Store will scan already installed packages, and if the app and version match what is in the App Store, it will disable the buy button to avoid a potential double purchase. It won't allow a re download through the store, nor will it update the apps. So for me, it looks like iLife updates will continue to come from Software Update, and 3rd party apps that the App Store recognizes will still have to be updated via other means.