Noddy Nodbox: I'm trying to do something like that, that's why I thought of the accelerometer. I have a Motorola Droid, but the same application dont work because when the screen turns off, the sensor data stops been broadcast, so the application dont work properly. This does not happen with all android phones tho.
Then I just thought about building one myself.
Thanks for the tip about IR Leds.
I'm not sure what kind of sensor would fit best, since I has to be precise (if possible, be able to catch breathing).