Apple is heavily promoting use of gestures, multiple desktops, and other OS features to promote fast switching and passing of data between running full screen apps. It's different than the pile-of-windows approach of the last few decades but the intent is still to have a ton of user-opened programs running and interacting. I think casual users genuinely prefer full screen apps, even after 20 years of pro users trying to lecture them and slap their wrists over the potential they're ignoring. Lion seems to be trying to give them their full screen, while also teaching a "safe" feeling way to multitasking and present a bunch of apps.
The trouble with mice is that they are a double proxy. You move the mouse in absolute terms, the mouse moves relative to a surface, and that relative movement is translated to the screen. The mind after a while abstracts away the mouse into a piece of your hand, so touchpads aren't really an improvement over mice, because you're just making relative movements on a proxied surface in both cases. In my experience the big usability issue with touchpads vs mice is that the area for relative movement is too small, requiring frequent repositioning. Apple gets it right by making the touchpad surface huge, so that you reposition your fingers less.
I expect that the current form factor is just an in-between until most computing devices look like ipads with external keyboards, with the mouse reserved for precision work (or perhaps we'll have dual finger/pen touchscreens).
Yes, mainly because we're moving away from the desktop. Lion is largely designed for iOS users and laptops.