You are all missing the point.
Look at the design of this device. It's aluminium structure mimics the Wireless Keyboard, which would suggest that they're meant to be placed together in the land of Apple design. It connects via Bluetooth like the Mice, which means it can be used with either a Desktop or Laptop.
Given how much stick Apple have had from their Mice over the years, I can honestly see the Magic Mouse as something of a forerunner to this Magic Pad. I would bet right now that this device will be released with the new iMacs come a month or two's time, with the Magic Mouse being the other option in the B.T.O menu.
Apple already give the user a choice of whether they would like a Magic Mouse or Apple Wired Mouse. I would suggest that with the new iMac and Mac Pro releases, the user will have the choice of either a mouse or trackpad, with the wired mouse being ditched all together.
I for one can't wait. The Magic Mouse is uncomfortable and lacks functionality of even the previous wired mouse. Imagine the possibilities that already exist in a MacBooks trackpad, except for your iMac. Most of those actions are instinctive; swiping, tapping and dragging. To those that say they "hate trackpads", you mustn't forget that this device is much larger. I'm sure it would provide a fantastic experience, and unlike Wacom Bamboo tablets, you use your fingers - not a pen!