I think I can offer a valuable example here. I've yet to post my GPA and MCAT score on this site, since I, like most of you, am pretty disenchanted with people who come on here to brag about how great they are and to ask us to assess their chances at getting into Harvard. But I think the stats would serve a purpose now. I graduated last May with a 3.97 and a 37Q on the MCAT. One summer was spent tutoring disadvantaged students on campus, one summer REU research program at UConn, and some other at least average EC stuff. Yet I received only 6 interviews after 19 complete applications, with only ONE ivy league interview (at Dartmouth, and despite having applied to every one of them). US News' entire top 20 quite literally just ignored me; my undergrad was a 4th tier public liberal arts college.
Don't mistake this for complaining. I know where I'm going already... I've been accepted to a school that I really love, and given the chance I wouldn't even choose any of the ivies over it. But...I think you see my point: I'm convinced that undergrad prestige can be a huge factor.