MacBook Pros run the Core Duo chip, which gets hot since it is damn powerful. Strangely the fans do not activate as often as they should (theory: Steve Jobs likes his Macs quiet) and while this doesn't pose a danger - it does leave the underside of a busy MacBook Pro pretty damn hot.
So, yes it is toasty underneath. The MacBooks are no better. All it takes is a future firmware update to get the fans on more often.
I am hearing things about heat, MacBook Pro and Intel's dual chips. I heard they are running at 125 degrees! Has anyone encountered this trouble? I was going to buy one today when a friend told me about this.