Why can't my cell phone function as a mouse?
It's got a whole bunch of buttons; I'm sure a few of them could act as mouse buttons (certainly if I hold it top down). The joystick thingie could even double as a scroll "wheel".
It speaks bluetooth like one of the natives, so communicating with the computer is not a problem.
It has a camera, and with the LED light/flash turned on it can see my desk just fine, though blurry, even when it lies flat on top of it. With the right macro lens, an application in the phone could detect movement by continuously taking and comparing pictures of the underlying surface, just like an optical mouse.
If I'm on the go, working on my laptop (if I had one, that is), I could be sure to always have a mouse handy, without carrying an extra gadget.
I could lie in my bed, watching a movie on my computer screen three or four meters away, using the phone to start, pause and do whatever I need to do without getting up.
Sure, a simple menu in the phone could be used to start and stop a movie, but the phone's display could show a closeup of the area around the mouse pointer, letting me navigate freely among my files, using menus etc. even at a distance where I can't see details on my monitor.
There are mice with phone features, so why not the other way around?
If you build it, I will come. (Not like that, silly!)