Something very similar seems to have already been done as an iPhone game now.
This is a very simple game mechanic which can be applied to multi-touch interfaces as well as keyboard and mouse interfaces.
Start off with a large blob floating in the middle of the screen. In physics terms, it's probably best thought of as looking down from above on a blob of liquid sitting on a teflon surface. While there is only a single blob on the screen, no points are earned. Using multi-touch, blobs can be separated by pressing on two separate points on the blob and pulling apart. On-screen, move the mouse pointer to where you want one finger to press then hold down shift (left or right), the pointer then splits in two, one stays where it was pinned and the other moves with the mouse. You can then click to hold down your second finger and pull apart.
Once the blob has been separated, the timer starts counting up points. The two blobs can now be let go and will float around the space freely, bouncing off the walls. If they touch, they will join again and the points will stop increasing. The player can grab hold of any blob at any time and hold it still so technically, they could just hold the two blobs apart and slowly rack up points. However, each of these blobs can also be pulled apart into two smaller blobs and for each extra blob on the screen, the score is multiplied. As before, if any two blobs touch, they join together and the multiplier decreases.
Blobs don't have to be the same size when pulled apart, holding two points near each other on the same side of the centre of the blob and pulling the outermost will pull out a smaller blob. This could be quickly repeated to create several tiny blobs which would be difficult to control but quickly increase score.
An alternative game mode can be introduced by having an increasing counter included in the game as well. After you have started scoring points, it will start counting up. If the counter catches up to your score at any point the game is over. The rate of increase can itself also increase as the game progresses