Why does my MacBook Pro die without warning?
Check to see how many cycles your battery has done (System Profiler - Power - Cycle Count); from what I understand your battery will still be under warranty if it has less than 300 - more than 300 and it's on its way out. We have a whole bunch of MacBooks (Pros) here, and once in a while one of them does this exact thing when the battery gets old and tired, just like yours. 

I've had batteries that have lasted 40 months, and some that have only lasted 18, the newer MacBooks with the non-removable batteries last a lot longer.