How many times has this happened? I found when I bought my iBook, the first time I ran down the battery I made it to 29% and then it died, second time around was 15-18%, third time was around 10%, etc. Now I can run it down to 3-2% before seeing a mandated shutdown. Of course, that doesn't mean I got all that battery life back, it just means the battery wasn't calibrated. I still only have 60-65% of the original capacity of the battery available, but after having run the battery down a half dozen times, I have a fully accurate battery meter.
Just a thought. Use CoconutBattery to find out how many cycles and how old the battery is. My battery only had 18 cycles on it but is 127 months old (10 years, woo!). Still runs for about two hours on a charge, which I think is insane. Standard life for a Li-Ion cell is 300 charge cycles.