Interesting. Why a stepper motor?
But anyway, you can get a DC signal that will indicate speed of rotation (as long as its fairly fast) just by rectifying and smoothing the output from one of the coils (http://en.wikipedia.org/wiki/Rectifier).
A single diode will be fine, but put a capacitor in parallel with the load resistor in the wikipedia diagram.
Select values of R times C to be significantly larger than the maximum anticipated period of rotation. R and C in Ohms and Farads.
Check it out and experiment with a scope or multimeter, to make sure that the maximum output is less than 5V for the fastest possible rpm and is smooth.
If its too high, then you may want to use a potential divider to drop it within range. Remember this adds to the load resistance.
If the signal is too small, then you will need to learn about opamps.
To be thorough, you should protect the analog input with a Zener diode. Search for posts on protecting inputs with a zener diode.