You know, the MacBook has such a beautiful trackpad. Are there applications that use its power for music-making, drawing, or anything else unusual?
In the "unusual" camp there's a program called Inklet that purports to turn your trackpad into a Wacom-like tablet. They also sell a special stylus and sketching software.
EDIT: I just now realized you meant software applications, not practical applications. Answering that question, there aren't any applications that I know of that have specific functionality built in for the gestures of the Magic Trackpad. There are lots of applications that allow for performing basic functions using trackpads, but nothing beyond basic scrolling and clicking gestures.
I use my Magic Trackpad exclusively and have found the gestures to be the most useful. As a drawing tool (I'm a graphic designer) it is not good...get a tablet for that. I have configured my gestures so that they invoke various tools or perform specific actions like creating or deleting a layer. There are keyboard shortcuts to performs these tasks, but I find a motion gesture to be faster than contorting my fingers to hit Command+Ctrl+N, etc.
Some of my global actions include: