As far as I know the only real way to do this is either inking it on the computer with a graphics tablet, or just the old fashioned going over it method. I do my outlining in MS paint with the curved line tool, and this is the kind of results it gets:
My gf uses Adobe Illustrator CS2 (She's a graphics design major). I recall her once telling me that it has an "Auto trace" feature that she really liked, as it allowed her to do her sketching in a pad, then scan the image in and have it traced.
She also use it to trace over manually at times, and to draw from scratch via wacom tablet. It's the industrty standard program so...
The most popular compy systems for colouring seem to be Adobe Paint and Paintshop Pro. Adobe has tracing tools you can customize that I've been told work well.