Greenman wrote this a while back, thought I would share:
Myths About Temperatures and Measurement
Myth: On-board temeprature sensors are accurate.
Reality: You shouldn't even ask what someone else's temps are. This is one of the biggest problems i see in forums. YOUR temps are not even accurate relative to your own system, much less someone elses. Let me elaborate. Just because your board reads 41C does not mean that if the temps drop 5C in real life the board will read 36C. Motherboards are not "absolutely accurate", nor are they "relatively" accurate. This is even worse with in-socket monitoring while watercooling. It is quite possible for the area around the socket to heat up to 50-60C, and influence the temperature measurements of the in-socket thermistor.
Myth: Digidoc5/ThermalTake/Vantec temperature monitors are accurate
Reality: While generally not as easily influenced by other factors, generally these sensors are off by more than +-3C. that means if your sensor is reading 3C high, and your friends is reading 3C low, you could be off by 6C on your comparison. More accurate sensors are available, the questions is whether that is important to you. The point is, do not believe onboard/digidoc temps. You shouldn't even tell other people what they are, they are so inaccurate. Tell them your overclock. There is very little inaccuracy in mhz measurements.