"Some MacBooks may have left the factory with a thin piece of clear plastic covering the rear vent," Apple admitted in the advisory posted last week. "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."
Symptoms of vent-blocked MacBooks include a constantly-running fan and heat coming off the top or bottom of the computer.
Owners of Apple's Intel-based notebooks -- the earlier MacBook Pro, particularly -- have complained for months of a variety of problems, including overheating. In May, Apple released a firmware update to regulate their laptops' fans, and lower the temperature of the machines.
Apple sent MacBook users with hot laps -- but no plastic stopping up the cooling vent -- to another online advisory on the Apple site. There, the Cupertino, Calif. company gives out advice such as "Do not leave the bottom of the computer in contact with your lap or any surface of your body for extended periods" and "The bottom surface and some areas between the keyboard and LCD hinge of your Apple notebook computer can become hot after extended periods of use. This is normal operating behavior."
Apple's notebook line wrapped up its migration to Intel processors three weeks ago when it released three MacBook models with 13-inch displays. Previously, it had introduced 15- and 17-inch MacBook Pro systems.