According to our service guys, the MacBook line has no "model number" identifier on the computer, itself. There is a "chassis number", but that can remain consistent across multiple models, so it will likely do you no good.
I agree with MBHockey, the Model Identifier would likely be the most consistent number across a model line. It can be located by going to the Apple Menu, clicking "About this Mac", then click "More Info", then click "Hardware", and look for "Model Identifier" in the Hardware Overview.
This number changes which each respective model release and would certainly help you track by model "number".
