I did some googling on this yesterday. It seems the back vent IS the main intake (air is sucked in through the sides and out in the middle). Macbooks should be constructed to operate while the lid is closed. Get a thin strip of cigarette-paper or tissue and whilst holding it about 2mm away from the vent-slot, move the paper from one hinge to the other. You should see that the paper is pulled towards the machine in one or two places (depending on number of fans inside) and pushed away at others.
That beeing said, openings in the chassis, such as the optical drive, also act as air intakes. As someone wrote on a forum - if you try covering the optical drive with tape and watch the temperature with iStat or similar program, you should see a 1-2 degree increase in idle temperature. The keyboard does not have much airflow, though heat is escaping the macbook by convection through the keyboard since it's the thinnest part (heated from below and radiates that heat upwards).
Conclusion: The keyboard is a minor air vent and mostly outputs some heat by convection, but covering it up should not make a huge difference. You'll probably see the macbook operate a few degrees hotter with a cover.
PS - I saw tons of people saying "the macbook only takes in air through the keyboard!" online, but none of them could back up their claims, they just "knew" this.. Don't listen to these people