TUAW brought to attention this knowledge base article on how to reset the SMC chip on your Intel-based MacBook, MacBook Pro and MacBook Air.

The System Management Controller is an integrated circuit (computer chip) that is on the logic board of the computer. As the name implies, it is responsible for power management of the computer. It controls backlighting, hard disk spin down, sleep and wake, some charging aspects, trackpad control, and some input/output as it relates to the computer sleeping.

So some possible symptoms when the SMC becomes unstable are your machine not turning on, waking from sleep, not charging or like the article from TUAW, the battery not showing the correct run time even though it was freshly charged.

But do note that resetting the SMC should be your last resort after restarting the machine to clear the problems. It did solved the problem posted in TUAW, hopefully it will solved yours next time.

(via TUAW)