Five years of tinkering, and I finally have a working product! It can be done!

Stats (exhaustive):
Dell Latitude P4 1.6GHz Laptop (from ebay - found one with a bad monitor)
Cantex weather resistant junction box - originally 12" x 12" x 6", plastic ~$50
Carnetix P1900 power & startup controller
Lilliput 8" LCD Touchscreen
16" bendable video mount ~$65

added 100G HD later, replacing the 50GB one that came with it
upgraded from windows 2000 to windows xp
Microsoft Streets & Trips 2000 with GPS adapter
logitech wired keyboard
Audio Advantage Amigo usb sound card
Goodsync Synchronizer


I wanted to have something more than an ipod in the car - a scalable system that could be modified and upgraded, and could hold my entire music library / mp3 collection without stupid apple or drm restrictions. I started by using a regular laptop with an inverter, but, yuck. Somehow, I discovered this website & community, and started planning for a more permanent solution - more than five years ago.
Over that time, I would focus on the project for a little while and then life would happen. When I picked it up again, the same roadblocks would be there and I'd have a new idea for how to accomplish...maybe you know the feeling...
Felt like I had all the necessary parts, and I finally started putting it all together last year - programming the laptop, developing the wiring, thinking through the final points of the design. A couple months ago, I realized that it was time to upgrade from my trusty 1995 Saturn SL-1; 235k miles and 43 states together, but it was time...
In comes my 2000 Nissan Xterra - the final catalyst to bring the project together.

Desired Functionality:
MP3 & custom playlists (free no-commercial radio)
MS Access database capable
DVD player
GPS navigation & trip tracking
mileage and maintenance tracking
web access?
PC versatility, laptop scalability and upkeep advantages
i could add a rearview / other camera (how cool is that!)
hide-able - not attracting excessive attention to the vehicle
hard-wired - no need/desire for laptop portability

Process?
I started off with the laptop - shopped for and bought off ebay, then stripped it down to the motherboard - took off the PC Card slots, the battery, and anything that could be taken off without impeding the operation of the computer. I eventually trimmed the metal skeleton frame so it would fit in the box, and fiddled with the BIOS to improve startup.
Bought the Lilliput, which seemed a bit large - thought I would mount it above the rearview mirror directly to the frame of the Saturn. (Even had a great wiring plan through frame holes behind the headliner, with a special extension cable for the video/mouse that you're welcome to buy off me cheap!) In the X it seems to work pretty well on the mount between the bucket seats. (Plus it's relatively easy to take off the mount - if the screen were smaller, it'd probably fit in the glove box, detached...)
When I started building the computer, I thought having a watertight box for the comp could come in handy - the original motivation behind the water resistant junction box. Of course, watertight = overheat. Also figured out pretty far along that the size of the box was impractical - even cut down, the mobo wasn't quite going to fit, plus I couldn't decide how to keep the box accessible for maintenance.
I ended up cutting the bottom of the box completely off. Then I cut an exact notch for the DVD drive to fit through, and drilled holes for air and cables. The beauty of the plastic box was definitely in easy modification without reduced strength. The base of the mobo case ended up being a piece of coated particle board that i gorilla-glued the top part of the junction box to. I might see about reducing the thickness of the board or replacing it. Maybe with some sheet steel or some modification of the former bottom of the box...
the lid of the box secures with 8 screws; I'll probably saw a couple off so they can be put back for appearance but not hinder on/off. As you can see from the pix, the assembly kinda fits under the floormat and passenger seat.
The usb hub was originally in the cpu - I decided against leaving it there so I could use the remaining USB port for whatever. Figured the increased air space could help too.

I bought the P1900 and started by locking the wires into a junction strip from radio shack - what can I say, I like order. When I bought the laptop, the seller threw in an aftermarket car adapter - i pulled off the laptop-to-transformer wire and connected it with coated copper to the P1900. Cut the transformer off the Lilliput power wire and connected it to the secondary (brown) output as well. On the recommendation of a Best Buy audio tech (who also gave me an inline 15A fuse for free) I got some 16GA wire and connected the P1900 to the battery. Ended up using more of the same (yes, excessive) to connect to ACC - after hunting around under the driver's dash for ignition wires, I ended up cheating and shoving the bare wire end into the ACC junction at the back of the factory stereo.
Connected it all up and got stuck with the laptop power-on setup (which I fixed with on-hand copper wires, solder and a radio shack momentary button switch.) Also, the monitor would not power up - read the codes off the P1900 and realized it was set up correctly -so i reversed the pos/neg wires from the monitor, it now works fine.
The computer (dell bios?) recognized the alternate power source and allowed me to turn off those notifications. I am thinking about installing the dell battery to assist with engine-off use, but based on what I've read here, it won't charge, will it... shouldn't need it anyway, the X battery is pretty friggin huge, after all..
I found a 50' ethernet cable at Best Buy for $20 that I'm using to network the computer to my home network router. The dell's wireless NIC may work, since my router is in a room next to the garage, but probably not without an antenna.

Issues & Questions Remaining:
audio connection to the car system: in the Saturn, I replaced the audio h/u with one that had a headphone jack - on the X I'm probably not gonna do this. at this point I have a Belkin universal radio transmitter, ($5 ebay) but it sucks - interference? Technically the X factory h/u has a tape deck but that adapter would likely suck more. Isn't there some way to jack into the antenna cable? Got some reading to do, but if you can help...
networking? is web-networking this laptop (on the road) worth getting an antenna?

Software:
I'm an Access geek. Way before the carputer idea, I developed a database that could catalog all my music, track legality, and build custom playlists. I modified the program so that it would have some clean, simple touchscreen interfaces.
I also like to track the mileage and maintenance information for my vehicles - with the Saturn, I used a home-designed book for the log - but with this system, I developed a touchscreen log program for the X.
I like winamp - so I made a large-button skin with transparency elements - so it can be used on the touchscreen without double-mode but not block what's sitting behind the player.
Microsoft Streets and Trips - i sought the GPS / trip planning userability of my parents' honda odyssey, but...um...Microsoft. (this is the biggest reason for the full size keyboard. brainstorming ways to make this more t/s friendly...)
Goodsync Synchronizer - I use this to auto-update the mp3 set, and the access databases for update (I do most of the programming and updating on my main laptop).

thanks for all your help out there! without this community, I'd probably have believed all the people that thought it was a crazy idea. if you're still on the fence about doing this, go for it!