simple way to measure wheel spinning speed with Adroid phone + magnet (and maybe Reed Switch)
OK, I am brainstorming a little bit here. I believe the easiest way to get rotational rate is to put something battery powered and blue tooth enabled on the wheel (preferably near the hub) with a 3-axis accelerometer. One or two of the axes will have a nice sinusoidal wave as the wheel rotates. This is an unmistakable signal. You can count time between max or min, or count zero crossings to easily calculate RPM's.

You will have to know the wheel rolling diameter to calculate speed over ground.

The smart sensor on the wheel will transmit data to the smart phone via blue-tooth. Could either transmit raw sensor data, or it could be processed into RPM's and only the end result could be passed to the smart phone.

You can also test it out with two smart phones. Have one act like the sensor, and the other like the display.