Wow, didn't really think I'd find a need to post this again, but from another thread where people were arguing which would survive a fall better between an iPod Nano vs. Classic and MacBook Air vs. regular MacBook....
Oh--and as for the iPod Nano vs. Classic and the MacBook Air vs. MacBook drop....
Yes, they will fall at the same rate. It doesn't matter which is heavier for that. However, the OP is right that the lighter product will hit the ground with less force. It's called impulse, and it's the change in momentum of an object resulting from integrating an amount of force F exerted during a given time t. That is, [delta]mv = int(F*tdt). Both objects will obviously hit the ground in the same amount of time t if we're talking about impacting on the same surface, and both objects will hit with the same velocity v, so in each case the object's mass is proportional to the force involved in the impact.
Bottom line: the MacBook Air has about 40% less mass than the ordinary MacBook, so it will be subjected to about 40% less force. Remember that in figuring out how the drop will effect the hardware.