"More information can be found in Intels Pentium 4 Thermal Design Guidelines (check out page 23)."
Several readers have submitted news of this clock-throttling, one aspect of the P4's built-in temperature sensor system Intel calls "Thermal Monitor." One thing to point out is that the same design guidelines document goes on to say that "the clock modulation feature of Thermal Monitor is disabled by default ... OEMs are expected to enable the thermal control circuit while using various controls and outputs to monitor the processor thermal status." Other things being equal (even if they never are) is there some reason to prefer a chip for not having this capability? If someone forced me to accept a free and loaded P4 system, I'd rather it be cool down at 750MHz temporarily than toast at 1.5GHz.