Ha! That’s funny—I made exactly the same suggestion to Jake (Archibald) today when we met up for lunch to discuss this very issue.
But he pointed out there would be problems with apps that using the “app shell” model where they take up 100% of the viewport height, but then manage scrolling for themselves between the header and the footer. In that situation, you don’t want the scroll upward action to reveal the address bar—you want to scroll upward within the content area.
I’m probably not explaining this very well, but long story short, this suggestion works well for sites like adactio.com but less well for app-shell sites that need to manage scroll events themselves.