I use paintshop pro, photoshop, and photoimpact to do my illustrations. Use the line tools to outline the vehicle, then color it in using the gradient tool or bucket fill. My images use about 4 layers to get everything lined up, but its well worth it. Takes some getting used too, but pretty easy once you get the hang of it. Photoshop Junkie has some great tutorials about everything relating to cars. Also, just go to google and type in cel shading tutorial and a bunch of stuff will come up I am sure.
I like the flash mx idea, but I don't think the lines will be as crisp. I'll have to try it to find out.