I remember anandtech did a article about that and it basically said that your better off overclocking your desktop processor than using an overclocked pentium M, considering how much the pentium M costs.
Just think if they started to make desktop mobos that supported pentium-m's. They are getting them to 2.5ghz on stock cooling w/o an increase in voltage. If they made desktop mobos for them, you could buy a low end pentium-m and up the voltage a little to get probably higher than 2.5ghz and you wouldn't have to be buying the $600 cpu. They use like 1.3v stock and getting nice oc's so with that little extra voltage (or more ) you could probably hit 3ghz on air i'm guessing. If 2.5ghz beats a fx55 than just think of 3ghz !!