Direct from Intel:
What is Intel® SpeedStep? technology? Does the mobile Intel® Celeron® processor have the Intel SpeedStep technology feature?
A: Intel® SpeedStep? technology is a feature found on all mobile Intel Pentium 4 processors - M and Pentium III processors - M at 600 MHz and above. Intel SpeedStep technology allows mobile Pentium 4 processors to operate in two performance modes: maximum performance mode when the notebook is plugged into an AC power source, and battery optimized performance mode when the notebook is running on battery power. Maximum performance mode maximizes the processor's operating frequency, while battery optimized performance mode maximizes battery life. Better yet, this switch occurs automatically when the notebook is either unplugged form an AC power source or plugged to an AC power source. No, mobile Intel Celeron processors do not have the Intel SpeedStep technology feature.
Edit: WCPUID does not report a change in the CPU speed when running on battery power for my ZE4000. Also, the 1.6GHz only disipates 30watts.