The 1ghz is Ultra Low Voltage, while the 1.2ghz is Low Voltage. According to sandpile.org
the typical powerconsumption of the 1ghz is 4-7W, while the 1.2ghz is 6-12W, that's quite a difference.
tvpad: can you force the CPU to lowest clock speed (600mhz) with Speedstep, and see if the fan stills runs?
I have it set for max. battery, and the clock speed is never above 600MHz. After about 20 min. of running close to idle the cpu temperature hovered around 38-39 C.
After starting a browser session with a large java applet the cpu temperature rose to 42 C for about 20 sec. This activated the fan. Once started it never stops, even after the system is left completely idle and temperature drops to 37 C.
It seems to me there's a problem. I wonder what software/driver controls the fan speed, if it's a bios issue or a defect.