I could be wrong, but afaik the P4-M only has a primitive system that will clock back on batteries and run full speed only on AC. Pentium M and Athlon XP-M (and 64) are lightyears ahead in this regard, since they dynamically adjust frequency and voltage depending on the load of the cpu (which makes a lot more sense). Unless Intel updated speedstep on the P4 and I am not aware of it, you'd be stuck with a (relatively) slow cpu speed on batteries even if you temporarely need the cpu power, and it will still burn too much energy by clocking way too high when its basically doing nothing (like surfing the web).
= The views stated herein are my personal views, and not necessarily the views of my wife. =
now realize, if this is for gaming, i do nto suggest you get any centrino package. i havent seen one taht had decent gaming performance, mostly becuase it isnt equipped with a good video card and its targeted more for thin and light and business that needs battery life. in the end, if your going to game on the road, consider buying 2 batteries.