Maybe you didn't mean it this way, but I read "killer app" and thought you must mean a program like Photoshop or something. On the contrary I think this would be an extension of OS functionality which uses many fingers and hands instead of just one mouse pointer.

The Finder's a perfect starting point. With a list of five files on your desktop, you could drag the second and fourth to the trash with two fingers of your right hand. With your left, you could simultaneously scoop up the rest and do a spring-loaded folders drag through the Finder icon deep into your home folder. So you could do all these simple file operations about as fast as you can think about them, instead of having to do them one at a time. Organizing your computer ought to be about as quick and easy as organizing a silverware drawer, and multi-touch begins to afford that.

I was watching one of those Google talks (like Merlin's Inbox Zero talk) which spelled out a new paradigm for how your computer should work: You have your content, and then there's functionality not nested within applications, but instead just floating out there and you grab it and use it and then do something else. An example the presenter gave was that you could start editing a PDF. You could edit your margins and clean up photo redeye in two quick, successive actions, all of about ten seconds. Today there are two ways to do that -- one, find a word processor that can edit photos or a photo editor that can do page layout. Two, use two programs and load the content into each in turn, saving and loading as you go. Both options are unwieldy; the former requires bloated programs with cluttered user interfaces due to too many featuers (see MS Word) and the latter requires too much time. In either case you end up waiting on something and proceeding at an unnatural pace. Now, neither of these actually depends on multi-touch, and indeed multi-touch wasn't mentioned in the talk, but this is the kind of different workflow that is going to be needed to really take advantage of doing things as fast as you can think about them.

So I think Apple will eventually do something like this, and actually I hope they do it right away. However I don't think that OS X is the right place for it. Mac OS was based on a mouse and a keyboard and, once you want to use multi-touch for serious work, that mouse and keyboard assumption is undermined and the desktop-with-applications paradigm no longer makes a lot of sense.