Most laptops have a DC input jack used to charge the batteries and provide power to the motherboard & peripherals. FYI most laptops have a DC-DC supply built into them already -- that's how they generate +5, +12, -12, and the LCD bias voltage from a single battery (10.2V for example)
I would still stick with a "desktop" model because of its ease of customizability (is that a word?!) and plus it's so easy to build a custom enclosure to fit in. I also like the desktop idea because the hardware is cheap -- replacing a desktop motherboard is much cheaper and easier than a laptop -- you practically have to replace the whole laptop if you fry the parallel port or something by accident.
Expansion is also nice with desktops -- just pop in another DIMM or replace the CPU without much trouble. In a laptop you need special memory and getting at the CPU is sometimes a problem.
Also, notebook hard drives are a lot more expensive than 3.5" drives, and some people don't want to pay $500 or so for a 18GB laptop drive to hold their MP3 collection when they can get a 27GB 3.5" drive for around $200. I may be giving an extreme example (I'm using a 6.4GB drive right now) but the point is the desktop hardware is so much cheaper.
The best thing about a laptop vs. a desktop PC is the laptop has a color LCD which could be detached and mounted somewhere for a really sharp, full-color display in a vehicle. Most desktop in-car MP3 systems have no display, an alphanumeric monochrome display, or a small 4" or 6" LCD that's not as clear as a true (S)VGA-capable LCD.
Anyway, that's my 2 cents, I'm sure others will follow with different opinions.