noncon has always been cut-and-dried to me -- it means, essentially, as you pointed out NO on someone's part.

dubcon kind of means the gray area of is-or-isn't it, where the NO may not seem as clear. I know this is making me sound like a douche, but dubcon is kind of a situation where a person says YES in a situation where it normally would've been NO (much like shinysylver's drunk sex example...)