In my rush to leave the country for Colombia (that really makes my life sound more interesting than it is), I inadvertently put my MacBook Pro to sleep instead of shutting it down. Two weeks later I returned. Waking it from sleep I found that the battery had become completely unrecognizable. The batteries in MacBooks need to retain a small charge to enable them to be recognized by the charging mechanism in the computer, completely draining them (such as by leaving them inside a sleeping computer for extended periods) will make them unusable. The lesson: Never put your MacBook to sleep for extended periods without power.
In my search for a way to reinvigorate my battery, I stumbled across a freeware application called coconutBattery, a great little app for getting detailed statistics on your battery. These include the battery load cycle (how many times the battery has been [almost] completely drained and recharged), the current capacity of the battery in milliampere-hours in comparison to its rated capacity and the age of your Mac. One bug I found though, it crashed looking for my dead battery, but works great for my new one!
I also wanted to mention that iStat Pro, a great general purpose system monitoring widget for Mac OS X shows the number of battery load cycles too.