I haven't heard of this happening in years, and even then, it only happened to like .000001% of Mac batteries (I think it happened to like 5 people out of millions sold). I remember my cousin got a free replacement battery for her laptop. That was back when Apple was using Sony batteries, before Sony fixed their batteries, and they weren't Lithium polymer, and they didn't have as advanced battery management software that the new ones use to make sure they don't overcharge.
My computer does run pretty hot. When I run flash on in Chrome on my early 2009 MBP, I get temps around 80C. If I run Starcraft 2 I get temps at 90C+. What's weird though is that my parents have almost the exact same computer, same time period, same user replaceable battery and all that, same processor, the only difference is that they have the 256MB Geforce 9600GT and I have the 512MB Geforce 9600T. Theirs run about 15C cooler. I am wondering if there might be some truth to the thermal paste rumor.