yes, this can be done, however,
I suggest custom designing a pc of sorts.
the problem is that you have to build it into the existing system,
but I'm sure you guys can figure out how to work an inverter to get +12 +5 +3.3v 0 -5 -12 v from the single 12v source,
thats all the computer needs,
so I suggest that you use a normal PC rather than a latop,
and I also suggest that instead of using a regular hard drive that you use a solid state memory such as a USB key, because although it won't break straight away, contsant vibration and road noise will shorten the lifespan of a hard disc.
Get a couple of touch screen monitors to stick in the back for internet access, lpug in a keyboard and away you go!!!
Either Alien ware, or Macs or anything else like that are going to (a) cost too much, (b) not off the flexibility.
Custom build your own, especially since you want to control other things in the car from this (can be done using the serial port).
Obviously if you have someone in the back with the laptop, you might not want then to control everything,
however with a custom built PC, you can have two monitors, (one for driver to control all the cars auxillary functions and perhaps navigation or GPS etc) and then a second pannel in the back for your passengers to surf the internet with.
Of course there is one last problem...
I assume you know that a moving car, (even with a 3G ready wireless card) you'll still only get GPRS speeds because 3G doesn't work well at speed...