You find such interesting things when you dig through the dark recesses of your file structure: hopes, dreams, ancient artifacts of great power, and even the occasional piece of unimplemented software. A blogger over at MyAppleGuide discovered, secreted away in OS X's Trackpad pane, an interface that would allow users of multitouch-capable trackpads -- such as those on the new MacBooks and MacBook Pros -- to define their own four-finger gestures.
The file's a .nib, which just means that it's simply the interface part of the equation--there's no code hooked up to it. But I've verified it for myself, and if you have a multitouch-capable Mac, you can find the same file at /System/Library/PreferencePanes/Trackpad.prefPane/Contents/Resources/ English.lproj/FourFingerSwipeGesture.nib (whew).
Currently, the multitouch trackpad's four-finger gestures are hard-coded: up shows the desktop, down triggers Expose, and swiping horizontally brings up the Application Switcher. The unused interface would have allowed for several other functions, including switching between Spaces and opening Dashboard.
Personally, I'd love the opportunity to remap these gestures. When I first started using the new MacBook, I constantly wanted to swipe up for expose, and down for showing the desktop, and while I've largely retrained myself at this point, I still almost never use the side-to-side application switching gesture, preferring instead to go for the keyboard equivalent of command-tab.
So, it looks like the configurable gestures may be coming in a future update, although perhaps Apple decided not to go in this direction just to keep it all standardized. My hope is for the former.