One of the major features of this WWDC Keynote was the fact that iTunes will be transitioning into the cloud. It’s already started, with the “Purchased” tab showing up in the iOS versions of the App Store and iTunes. It works exactly like they showed, allowing you to view everything you have ever purchased or just what isn’t on your iDevice.
There’s also what I presume to be the new iCloud download icon. Like I mentioned above, it works for both the App Store and the iTunes Store on your iPod touch, iPhone, and iPad. It goes all the way back to the first apps you’ve every downloaded, which (for me) was day one of the App Store. The first app I ever downloaded? Oh, that was WeatherBug. Then Lightsaber Unleashed. Ah, memories.
You can find the “Purchased” tab under “Updates” in the App Store, and in the iTunes Store it’s even more obvious.