"Do not leave the bottom of the computer in contact with your lap or any surface of your body for extended periods. Prolonged contact with your body could cause discomfort and potentially a burn."
That's Apple's cautionary advice about allowing its notebooks to touch your person, but the problem isn't just a Mac thing. With all the powerful technologies packed inside today's notebooks, the bottoms of many portables get hot, hot, hot. Long-term, using a laptop on your lap could be hazardous to your health--or at least your comfort.
What to do? One option is to use a lap desk, like those available from Levenger. Such desks not only put some distance between lap and laptop but provide a flat surface that helps a notebook keep its cool.