A little known fact about the unibody touchpad is that it can distinguish between up to eleven touch points - i.e., fingers.

Another little known fact is that the sensor knows about the contact surface area (in terms of ellipsoids).

Another yet less known fact is that Johan Nordberg didn't sleep much last night, and wrote a neat OS X application which simply plots what the trackpad (or Magic Mouse!) is saying about touches.

Download blä.se/FingerMgmtUB.zip to try it out.

Update: Apparently works on older MacBooks as well! Give it a shot, it might be your lucky day.