Here's what I think... They get rid of the black box, and release a small HDMI dongle/receiver that picks up AirPlay from your iPad. On the iPad you have a new TV app that has everything and more the old Apple TV had.
The iPad IS the new Apple TV!
That is an awesome idea. Doesn't explain why they'd discontinue the Apple TV 2 though, since not everyone is going to buy an iPad 3 immediately - or ever. They'd miss out on a lot of sales by discontinuing the set-top box.