While everybody was gawking at the news coming in from the WWDC 2011 Stevenote today, Apple silently slipped in an update to the iOS iTunes and App Store apps. The update, which was made on Apple's servers, adds a feature that was described in the keynote -- the Purchased button.
As you can see in the screenshot at right, looking at Updates in the App Store app now displays a Purchased button. Tap on it, and you see a list (below) of every app you've purchased. There's a tab displaying all purchased apps, and another showing just those that have been purchased but are not currently installed on the iOS device.