Okay, I have commented on this in a few places, but today I decided to test it to say for sure.

For starters, feel free to read the Wikipedia article regarding the Athlons, which has this to say about the Mobile versions:
Mobile Athlon XPs (Athlon XP-M) are identical to normal Athlon XPs, apart from running at lower voltages and not being multiplier-locked. The lower vcore ratings allow the CPU to run with less power consumption (ideal for battery-powered laptops) and produce less heat. They are also capable of having their multipliers dynamically adjusted by software to supply faster speeds at higher frequencies when demanded, but throttle back to lower speeds and voltages when CPU demand is lower.
So the two advantages are:
1) ability to run at lower core voltage for lower power use
2) ability to change multipliers on the fly for lower power use when CPU load is low.

Now, #2 is something that I have never seen successfully implemented on a desktop board; I assume it is a feature that is specially added when they are building laptop motherboards from the ground up. With some motherboards (ie - Biostar M7NCG) it is possible to change the FSB on the fly, however I have not seen this method capable of throttling back the processor more than about 33% of normal speed; changing the multiplier as well would be much more significant if it were possible.

#1 is the one that I really mean to address. The stock core voltage of an athlon XP-M 2200+ (and a number of other XP-M's) is 1.35v. Some motherboards with BIOS over/underclocking features may allow you to reach this voltage (such as the Biostar M7NCG 400) so the CPU consumes less power while still running at its full rated speed.
However, many motherboards with chipsets made by VIA, SiS, etc do not support multiplier or Vcore adjustments in the BIOS. These boards fail to recognize the mobile processors for their true speed, and as a result you need to pin-mod them so the motherboard detects the right multiplier to use with the processor. (search the forums if you need info on pin-modding) The issue there is that the standard pin-mod only allows you to get the core voltage down to 1.475v, which is a lot higher than the 1.35v the mobile processor should be running at.

The point is, since (so far) we don't really have access to the advantages of #2 above, and if you are pin-modding your mobo to use an athlon XP-M you are losing much of the advantage of #1 as well.

To test this out, I started with my Biostar M7NCG 400 mobo, my athlon mobile XP 2200+, and my regular athlon XP 2500+. I cut the 'hot' wire of the AC power cord to the desktop AC-DC ATX PSU I was using to power the system, and placed my DMM in series to measure AC amps. (disclaimer: Please don't try that at home if you don't know what you're doing, it's 120VAC so it can be dangerous) That gave me the overall power draw of the power supply and thus the entire system, other than inefficiency, but I was only looking for relative values anyway. All current measurements were done with the system sitting at the main BIOS menu.
The mobo supports multiplier/FSB/vcore adjustments in the BIOS, so I set them at the stock settings for the athlon XP-M, at 1667 MHz:
133 FSB, 12.5x multiplier, 1.35v core, mobile athlon XP
results: 440mA, ie - 52.8 watts
Next, I turned up the vcore to the regular XP stock voltage:
133 FSB, 12.5x multiplier, 1.5v core, mobile athlon XP
results: 500mA, ie - 60 watts
Then, I swapped in the regular athlon XP processor; all the settings were left the same, so the system still recognized it as a 2200+ running at 1667 MHz
133 FSB, 12.5x multiplier, 1.5v core, mobile athlon XP
results: 500mA, ie - 60 watts

I suspect that the difference between the 1.35v and 1.5v core voltage power consumption would be much larger if the system was under a heavy load, I can't test that until I install an OS though.

But, the main point is that the mobile and regular athlon XP processors use THE SAME amount of power when they are run at the same speed and core voltage. So, if you are going to be using a motherboard that requires pin-modding to recognize the athlon XP-M (which is most of them), you most likely won't be able to reach the 1.35v core voltage, and thus you might be able to save yourself the trouble and use a regular athlon XP, many of which can run fine at the 1.5v or so that the pin-modded boards can manage to reach (especially since you shouldn't be overclocking a carPC)
If you already have the mobile processor, then it doesn't really matter much, you can pin-mod it and it won't be any worse than a regular athlon XP, but don't buy one expecting it to use less power unless you have a mobo that can do 1.35vcore... and from what I have seen, there are not that many micro-ATX mobos that can do that (I know of none other than the M7NCG)

Anyway, I'm not sure how useful this is, since athlon XP's are about obsolete so prices can be a bit weird, so the mobile XP's may very well be cheaper in some cases... But I just didn't know how many other people realized it, given the popularity of the mobile XP's on here, and it may be useful for some people who are considering making the switch to a micro-ATX board but are dreading having to do a pin-mod.