From the horse’s mouth
Apple – Software Download (direct)
You may have just bought the Retina-based MacBook Pro or a Macintosh laptop over the past year but have noticed that you always have to run it on AC power rather than on battery. In some cases, you may have noticed that the fan in the computer is running more than it should.
This has been discovered by Apple as a firmware bug and they have prepared a software update to rectify this problem. This bug is allowing the CPU in the MacBook to run harder than it really needs to, thus allowing it to draw on more power and yield more heat. The software update also improves operating-system compatibility with certain USB devices which may also help with the stability of the system.
This software update is available at Apple’s Website or through the Software Updates panel in MacOS X Lion. Infact, if you have just bought a Mac laptop, I would recommend that you visit the Software Updates option and deploy this and other important updates to your Mac.