[pasbesoin]

I haven't used it for your type of application, but I've used Potrace to convert some scanned hand-drawn graphics to SVG. The results were quite satisfactory.

http://potrace.sourceforge.net/

As I recall, I ended up using it via the front end Rasterbater 1.0 (linked on the above page; and as for the name, I know... but it's legit; have a look at http://en.wikipedia.org/wiki/Tiled_printing ) because it was the best/quickest compromise at the time. I also recall reading that people have used Potrace with Inkscape (a very good cross-platform SVG editor) in order to convert color images to SVG; I recall that workflow as something like using Inkscape (well, that doesn't quite make sense? I dont' remember, I guess) to produce color separations and then running Potrace against each separation.