UPDATE (Jun/7/2009) : I've played a lot with undervoltaging my HP TC1100 and IBM Thinkpad t42p as is explained at
http://forum.notebookreview.com/showthread.php?t=235824 (and recommended in the above-linked TC1100 Wiki).
I have GREAT results. I've managed to decrease the voltage with more than 0.2V on the Pentium M Dothan-based t42p and slightly less on the Pentium M Banias-based TC1100.
On the TC1100, the cooldown advantage I've managed to get is about 2C degree at full CPU usage. (The TC1100 uses the Pentium M ULV CPU, which already is, as you may have guessed, running on lower voltages.) Nevertheless, this already means the fan "kicks in" far less often. With a protective case on (which doesn't let the TC1100 cool down, except for the two fan openings) and only browsing Web pages with Opera, without the Flash plug-in (which is a real CPU hog, even when running in the background; this is why I don't let Opera install the Flash plug-in but just copy the URL of a page I want to see its Flash content into IE) installed, it only started after about 30 minutes. Without the case, the fan has never kicked in.
I've got even better results with the t42p. The CPU running at 2 GHz and 1.32V, the maximum temperature I recorded was 82C and the average 77-79C. The CPU undervolted to 1.1160V (which is two selectable values more than the already-unstable [Orthos displays a computation error] 1.0840V; 1.1000 would have resulted in 60C), I got the maximum of 61C. That is, about 20C less than in the default case!
I was pretty lucky with decreasing the minimum voltage (at 600 MHz). Instead of the default ~0.98V, I've found out 0.7320V is still sufficient. (Going even lower, 0.7160V, has resulted in Orthos displaying a computation error.) VERY impressive!
Note that, under Windows 7, when returning from hibernation, it will auto-start in the background. It also displays an icon in the lower right corner of the screen (hidden by default in Windows 7). It might be useful to disable it (right-click and Hide) to further reduce CPU usage.
I can only recommend the RightMark overclock app - it will REALLY cool down your notebook. The CPU consuming considerably less power (and dissipating far less heat) will not only resulting in major battery life savings, but also a much more silent notebook. And, of course, it's easier to work with a notebook that doesn't overheat - even if fan noise or higher power consumption isn't an issue with you. For example, the IBM Thinkpad t42p has the fan in the lower right edge of the notebook (unlike, for example, the, in this regard, better-designed IBM Thinkpad a31p, which has the same in the upper right edge). The notebook running a CPU-intensive task makes working pretty hard: everytime you let at least part of your hand drop from the notebook, you'll almost burn it in the stream of the hot air. With undervoltaging in place, this effect is far less pronounced - after all, the stream of air itself is about 20 degrees cooler.
Three screenshots of my TC1100 and t42p undervoltaged. Note that, with the TC1100, I fine-tuned all the voltages - not only the highest and lowest (for 1 GHz and 600MHz, respectively). With the t42p, I used the automatic computation, based on the manually-set and checked highest and lowest frequency values, compute them. Of course, I've tested their stability (as on the TC1100) by disabling the highest-frequency checkboxes in turn, one after another.
TC1100
T42p
Still another remark: if yours isn't a Core (2) Duo CPU (but, say, a Pentium M Dothan or Banias), then, in the "Advanced CPU settings" tab, there will be almost nothing you can en/disable, unlike with later CPU editions.
All in all, I can only recommend undervoltaging. It's well worth the effort and time.
Some other benchmarks I've made, showing how the heat buildup depends on the voltage:
At 2 GHz:
1.2280V: 68C
1.1800V: 66C
1.1480V: 64c
1.1160V: 61c
1.1000V: 60c
At 600 MHz:
0.7800V: 43c
0.7640V: 42c
0.7480V: 41c
0.7320V: 40c