I am also wondering if I can mill mild steel with mine. I am close to having mine built. It's a gantry style, hoping for it to be pretty versatile. See pics on my post: My machine, need motors and controllers - CNCzone.com-The Largest Machinist Community on the net!
I have a porter cable 690LR router on it with 1/4, 3/8, and 1/2 inch collets. I suppose it's just a matter of finding the right mill bits. I'll be interested to know what you find out about that.