On a page dedicated to maximizing your MacBook's battery life and lifespan, Apple says:

Apple does not recommend leaving your portable plugged in all the time. Apple recommends charging and discharging its battery at least once per month. Need a reminder? Add an event to your desktop's iCal.

Apple also says that you should store the battery with a 50 percent charge if you're not going to use your laptop for more than 6 months (otherwise it could fall into a deep discharge state, and not be capable of holding a charge).

Many laptops—PC and Mac—use Lithium-Ion batteries (as apposed to Nickel-metal hydride), so it's a good reminder to make sure you read your freakin' manual. What's been your experience with laptop batteries? [Smarterware]

Meanwhile, Dell responsibly reacts to the news of its laptops catching on fire (see photo above) with a huge recall involving multiple millions of lithium-ion battery units, while at the same time multiple thousands of these batteries are still flying in airplanes as you read this. That recall may not happen soon enough. Note to TSA and Dell: "Dude, you're getting into a fiery plane crash." – Charlie White

Laptop batteries on planes are an accident waiting to happen [The Inquirer]