p.s. the site is free but if you use it a lot consider making a donation
Update:
The Rasterbator site is awesome but I want to use this technique for a larger project and I needed more consisten results. So I wrote my own halftone program that produces SVGs directly. If you know java here is the source, feel free to tinker:
OMG I have to tell you I just now clicked that link this is the app I had downloaded on my computer when I was in middle school, I have been looking for this for so many years!! thanks!
usage: java -jar halftone.jar [options] "path to image"
--dpi= Dots per inch. I recommend 10-15(default 10)
--no-border No border cut line will be added