I have the same motorshield and the same dilemma. my plan is I ordered the stacking pin kit from Adafruit and am going to change out the pins on the motor board to hookup other pins/sensors to.. The board is basically the same as the Adafruit version 1 motorshield, just a Sainsmart clone. i f you look really close on the shield you will find that their is a small breakout board area on the board you can hook up to for pins A0-A5. but they are really small.
Go here for some pretty good info. Overview | Adafruit Motor Shield | Adafruit Learning System
Here is a really good pdf for the shield too. http://learn.adafruit.com/downloads/pdf/adafruit-motor-shield.pdf
Also make sure you use the AdAFRUIT motor libraries they are the best ones for that shield.