My P4 2.26 using the stock Shuttle ICE heatpipe sits about 46 when idle with the fan at quiet speeds, and it drops to 38 with the fan running full blast. Under load, CPU usually sits in the 50's. 68 is my threshold for the fan to kick into full speed mode, and it has never done this on its own.

--edit And for the record, all P4 chips have had integrated diodes to measure their temp. This is how they implement the emergancy clock throttling if the processor overheats. Athlons, the initial ones didn't have a diode, but the recent ones do (Athlon XP, etc...).