I hit that once too, encoding to h.264. It would get around 75 just watching flash vids on YouTube. My fans were a howlin'. My MBP wouldn't even break a sweat over those tasks, but the MB really gets a work out.
My fans kick in big time just looking at youtube videos. Which never happened on my old G5, with a worse video card. Thought it was strange to see the macbook heat up like that but I guess it's common then