It will run hotter and suck more juice. While that's not a problem if you are running it plugged in most of the time, it will result in shorter battery life per charge, and the increased heat may cause other problems. But again, if you are using it in a plugged in situation mostly, it won't matter.
The P4M has Speedstep, or whatever Intel calls it. Pretty much it's the shutting down of unused processor parts, selective gating of certain areas that are unused. So if you are gaming or playing a DVD, most of that processor will be used, but if you are just typing in Word, most likely 90% of the proc will be idle, and therefore "turned off." Most notebook makers that insist upon using the desktop versions of the P4 use a software-based throttle, such as the Dell's, who have 100% and 50% (effectively the processor's clock speed is cut in half). The down side to this is that it happens no matter what when the computer is running on battery, unless you specifically tell it not to, whereas the speedstep stuff is dynamic, and adjusts on the fly.
Some day, THG-willing, I shall obtain the coveted "Old Hand" title.