A Pentium M (Centrino) processor will perform somewhere between 1.5x and 2x in comparison to a Pentium 4M (notebook version of Pentium 4 desktop processor).
The mhz is dramatically reduced (as Athlon 64 cpus and Apple cpus are in comparison to a Pentium 4/4M) to give better battery life, allow a thinner and lighter notebook design (as less space is needed for fans/ventilation as cpus run cooler), etc.
Two of the things that allow the Pentium M (Centrino) to run faster in comparison to the Pentium 4 is a large amount of L2 system cache on the chip (which is much faster than system RAM) - the new Dothan processors have 2mb L2 system cache -- and the chipset on the motherboard for Centrino Pentium M's that is optimized for wi-fi and other tasks.
In the near future 64bit chips will arrive with 2 cpus per chip that will allow true multitasking (burn DVD using 1 cpu and watch movie on the other cpu, etc).
The Athlon 64 chips now will run better with 64 bit in the future but they only have 1 cpu now.
The Pentium M or Athlon 64 doesn't have hyperthreading to improve multitasking somewhat but that enhancement is negligible especially if you are not running multiple heavy processes all the time simultaniously.
A Pentium 4M is still a good choice if you want a large 17" LCD notebook that has a 128mb dedicated video card for gaming -- like the hpzd7000 or Toshiba or Sony equivalents) but for 15" or smaller notebooks the Pentium 4M or Athlon 64's are better for true notebook use (battery life, weight, transportability).
And, you can get 128mb dedicated video on a Pentium M notebook like the Dell 8600, Acer 2000 series, etc.
There is so much choice out therer? How do you compare the speed of a processor like a mobile pentium 4 M? The MHZ seems to be signifigantly lower, like 1-2 MHZ comapred to a 3.6 desktop or mobile intel pentium with HT? Whats going on? and why is there no tablet pc with anything more than 1.6 mhz?