It happens more often with my road bike than with my CX bike, oddly enough. Roadie has a much more sloped top tube, whereas the CX is a more traditional geometry. I don't really care. I just scrub it off when I get home. (The cleanser I use on my face works quite well, actually.)

You can use Simple Green on your clothes, but you run the risk of getting your pants caught in the chain. Much easier just to scrub your leg off.