Like virtually all mobile computer manufacturers, Apple recommends that users calibrate their laptop batteries every few months, as Scott pointed out a couple of years ago. Unlike many manufacturers, however, Apple does not include a utility to automatically perform a calibration. Given Apple's uncanny ability to make things user-friendly, this seems like a bit of a glaring oversight.
What are the benefits of calibration? Primarily the goal is to ensure that the microprocessor in your battery provides an accurate estimate of how much time you have left on the battery. Over time it can lose track of just how much juice your battery has in it, and calibration gives it a very accurate reading on the battery's health.
Older battery technologies also benefited from a process called conditioning, which was typically performed the same way as a current battery's calibration process. In a nutshell, you fully charge the battery, fully discharge it, then fully charge it again. It's unclear whether modern lithium batteries gain anything in an actual capacity perspective from this process, or if it is simply a calibration of the microprocessor for the purpose of providing accurate time estimates.