Gee, THANKS for all the great comments.
Tommy, I used to use Illustrator, but I now use CorelDraw! 11.0 - I've found it to be a little easier to import/export into other formats. It is also better for large printing. I have printed a few of these images as large as 3' x 6' - so the rendering needs to be really smooth at that size.
P.S. Here is another drawing I did recently for a friend who works at USAirways.