In fact, Apple's MacBook Pro User Guide explicitly states "Do not leave the bottom of your MacBook Pro in contact with your lap or any surface of your body for extended periods." Adding that "prolonged contact with your body could cause discomfort and potentially a burn."
According to Kyle at iFixit the MacBook has at least two internal temperature sensors: one glued to the heat sink, and one to the lower case on the right hand side. You can see them in the upper right-hand corner of this picture. They're small rectangular green chips Kapton'd to the enclosure, with red/white wires running to the logic board.
None of the various temperature reading applications have been updated to read the sensors in the MacBook Pro so I took matters into my own hands. The following temperature benchmarks compare a MacBook Pro 2.0GHz and a PowerBook G4 1.5GHz, both configured with 2GB RAM and 120GB, 5400 RPM hard drives. I played a DVD on each of the machines overnight and measured the temperature (using a Cooper Atkins 313K digital thermometer with a surface probe) in the morning.
Temperatures were measured across the top of the keyboard, from left to right above the F1, F6 and F12 keys and across the bottom of the machine (again from left to right) along the edge closest to the hinge.
PowerBook G4 1.5GHz:
Top - 112, 114, 116 (degrees Fahrenheit)
Bottom - 109, 103, 101
MacBook Pro, 2.0 GHz:
Top - 108, 108, 106
Bottom - 108, 112, 109
The MacBook Pro ran 4-10 degrees cooler than the PowerBook G4 on top but on the bottom it was almost 10 degrees hotter than the PowerBook G4.
I consider these results to be mixed and although unscientific, they provide a barometer for comparison. It should be noted that the MacBook Pro can get much hotter at times, I measured top temperatures as high as 126 degrees Fahrenheit. As always, your mileage may vary.
What is your preception of the heat generated by the MacBook Pro? Is it more or less than your previous "notebook?"