I had never tought of having an CCD camera in my optical mouse.
But now after reading this site (http://spritesmods.com/) I was suprised.
Actually the optical mouse contains two chips inside: the optical sensor and a chip for the usb/ps2-interfacing. The optical sensor is a tiny 18×18 CCD. Using this sensor we can see small images. The author has written a small app to see the images on the screen through the parallel port.
See this link to find more about this here.
Check his site for more hacks http://spritesmods.com/