Wired has an article regarding the temperature of laptops and what can and will be done to bring it down.
Laptops largely rely on heat pipes that use liquid coolants to cool and distribute heat away from the chipset. Fans also keep temperatures down, albeit on a smaller scale compared to desktops. However, the resulting designs see varying degrees of success.
"In some rare instances they do a really good job -- the larger the laptop the easier this is," said Rob Enderle, principal analyst for the Enderle Group. "But often either the bottom, the keyboard or the wrist rest become uncomfortably hot even with the latest processors."