There are so many posts about the heat issues with the MacBook I thought I would post some comparisons of other Mac hardware for comparison. I only repeated the test twice and averaged the measures. I'm sure your mileage will vary.
So I've tested my 1GHZ Aluminum Powerbook and my 20" 2.0GHZ dual core iMac along with my MacBook.
76 F Ambient
G4 PowerBook idle : 106 F
G4 PowerBook full load: 141 F
20" Dual core iMac idle: 101 F
20" Dual core iMac full load: 133 F
13" Black MacBook idle: 124-126 F
13" Black MacBook full load: 179 F
Clearly, Apple's notebooks run hotter than its desktops, although one could argue the iMac is just a really big notebook case. Higher heat in portables makes sense given the challenge of removing heat from a confined space. However, at idle, the Macbook is only 20 degrees F hotter than a G4 or the iMac. It does get warmer under load than either of those machines, however, at about 40 degrees hotter than either one. Actually, almost 50 degrees hotter than the iMac.
Since the iMac is using the same CPU as the MacBook, this temperature difference should be entirely dependent on the ability of each to remove heat from the enclosure. The iMac is much better at doing this than the MacBook (I won't speculate as to why/how).
The iMac keeps itself cool with no problems and was the fastest to drop CPU temps when the load was removed from the CPU, shedding virtually all of its surplus heat in 30 seconds and returning to baseline. The PowerBook felt uncomfortably hot underneath near the back and also the screen hinge was REALLY hot during this test. I also heard the PowerBook fan come on, but the air volume from this fan is significantly lower than the MacBooks fan (which also came on).
I found the MacBook and older PowerBook to feel equally warm against skin, most likely due to the aluminum's physical conduction being so high and the MacBook's polycarbonate conduction being lower. Part of the reason the PowerBook runs cooler could be its use of its entire skin as a sort of heat sink.
In any case, while the MacBook does run hotter (and significantly hotter under load), I found it very acceptable and certainly not any worse than the older PowerBook.