"Surfin the net" in the car isn't the real goal here, it's just simply having connectivity. I'm a software developer with a creative imagination... there's lots of possibilities with an online "car-puter" always connected to your house via vpn.
For example, I've got security cameras around my house I can tap into and check to see who's home, or if guests have arrived before me. The 3G connection speed isn't going to be the same as cable, but I'm sure it would suffice.
Regarding the laptop, I wasn't thinking of just leaving it on the seat, rather mounting a small ventilated metal housing in the back that could hold a Dell Inspiron Mini 9 (9"x6.7"x1"). I do see the logic in installing one of the "brick"-like computers meant for vehicles, but the downside is it would have no use outside of your car; a laptop can be removed to take to a meeting, etc etc.
There's lots of options to consider... the great thing is there's no shortage of solutions/products for this sort of thing!
Thanks for your input, opinions & links! Sometimes the things we do for our cars aren't too practical, but dangit they're lots of fun!