when i'm home i prop up the back of my ibook with an old dictionary about 3 cm thick. i notice the fan comes on less often this way. that's why i do it.
when i go mobile, it's not an issue. for me, the only time the machine (gpu i guess?) is taxed enough to trigger the fan is when i'm watching a movie, gaming or photoshopping, none of which i do on the road.