The battery on my 15" MBP (pre-unibody, bought Jan 07) has completely died. It expanded and looks like its about to pop. I took it to the Apple store, and they said that this was a "safety feature" to let you know when it's time to replace the battery. Total BS I'm sure, it had only done about 120 cycles.
Sounds like you were told a FIB by a sales person. You know the ones standing at the front of the store.
The geniuses are the "Techie" guys. Trust me.