iPad runs iPhone apps too. Sometimes people want to make it bigger and they click the "2x" button. Then the OS scales the app with pixel Doubling, which makes it look ugly because the whole app is pixelated.
Now that iPhone apps can be rendered at doubled DPI on an iPhone 4 with crystal clear quality, why doesn't Apple use this mode to display iPhone apps in 2x on an iPad? Is there a consideration other than time constraint?