MacRumors has uncovered a intriguing patent application from Apple describing some of their possible planned multi-touch interface additions to OS X. Combinations include four finger taps to reveal Exposé, thumb-and-finger combinations, and even a way to recognize the pinky at the bottom of a fist for some sort of club-handed command. (I'm guessing why-you-little-Ctrl-Z.)
Whether or not we'll see any of this stuff on current-generation hardware (besides the Macbook Air) is anyone's guess. I think it's at least possible these upgrades could be done in software, despite some debate on whether or not the chipset powering the trackpads could handle it. I base this purely from the increased size of the trackpads from the PowerBook to MacBook. Surely they wouldn't have nearly doubled in size just for two-fingered scrolling?