Great information from Apple regarding how to maximize battery life in your Apple notebook, iPod or iPhone:
For proper maintenance of a lithium-based battery, it’s important to keep the electrons in it moving occasionally. Apple does not recommend leaving your portable plugged in all the time. An ideal use would be a commuter who uses her MacBook Pro on the train, then plugs it in at the office to charge. This keeps the battery juices flowing. If on the other hand, you use a desktop computer at work, and save a notebook for infrequent travel, Apple recommends charging and discharging its battery at least once per month. Need a reminder? Add an event to your desktop’s iCal.
For Apple notebooks with removable batteries — such as previous generation MacBook and MacBook Pro computers — a properly maintained battery is designed to retain up to 80% of its original capacity at 300 full charge and discharge cycles — referred to as Cycle count in Apple System Profiler (look under Hardware > Power > Battery Information: Health Information)
The built-in battery of the MacBook Air is designed to retain up to 80% of its original capacity at up to 750 full charge and discharge cycles.
The built-in battery in the new 13-, 15-, and 17-inch MacBook Pro is designed to retain up to 80% of its original capacity at up to 1000 full charge and discharge cycles.