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After some users discovered that the back vent in some MacBooks were blocked by plastic, and then subsequently blogged about it, Apple's released a somewhat official statement about the matter through their Support Site.

Some MacBook computers may appear to be running too warm, with the fan running consistently and heat emanating from the top and/or the bottom of the computer.

If this happens with your MacBook, check the rear vent of the MacBook to make sure it's not blocked.

Some MacBooks may have left the factory with a thin piece of clear plastic covering the rear vent. This is used in the factory to prevent dust from getting into your computer. If your MacBook has the plastic still over the vent, simply remove and discard it.

MacBook may run warm because rear vent is blocked [Apple - Thanks Jonathan San Francisco!]