One thing that I noticed when Mac OS X Lion was being demoed yesterday was how difficult of a time the demo person was having utilizing the multi-touch gestures on the Magic Mouse. Take a look at the video from yesterday, you can clearly see that when he is trying to perform the swipe to the side to change to and from a fullscreen app he has to do it a couple of times to get that gesture to take. Not every time, just some times.
It is also important to note that this is a guy who has theoretically spent a lot of time practicing this very gesture.
Ben’s right — this was an awkward moment during the keynote. Without a doubt, the Magic Trackpad was invented with Lion in mind. Apple is the most methodical company on the planet — there’s nothing done without a reason. I’d bet that by the time Lion rolls around, iMacs will ship with the Magic Trackpad — not a mouse — by default.