Some MacBooks have been inadvertently shipped with a clear plastic sheet covering the rear vent, causing them to run too hot. If your fan runs consistently, and heat emanates from the top or bottom of your laptop, the vent may be blocked.
From Apple’s Knowledge Base article:
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.
The vent is positioned at the back of the laptop case where the lid hinges sit. Even if your MacBook seems to be functioning normally, it’s still a good idea to check and see if the rear vent is blocked.
[Thanks to AppleInsider for pointing out the KB article.]