There was actually an interesting study about this which got mentioned recently on the Freakonomics podcast (you can see a transcription of the segment at http://freakonomicsradio.com/does-college-still-matter-and-o..., the original study is at http://faculty.arts.ubc.ca/tlemieux/papers/unintended.pdf).

Basically, it uses men who were eligible for the Vietnam draft (who enrolled in college in greater numbers than they otherwise would have, to avoid military service) to demonstrate that success from college graduates is more than a "these people are the ones who were already going to succeed" thing.