Unfortunately,
I think it does matter where you go for undergrad (to some extent). I can't help but notice how many of my classmates went to Stanford, Berkeley, Princeton, Harvard, Yale, U of Chicago, UCLA, Brown, Yale, etc. Yes, there are some that went to 'lower tier' schools, but the majority of my classmates went to very well-known and highly 'ranked' schools.
I do believe that someone with great accomplishments can get in anywhere, but I do think the your undergraduate reputation does give you a little bit of assistance. I think the same thing goes for residency. Someone who kicks ass on the USMLE and in their clinical rotations will land a sweet residency, but I think those of us that go to highly ranked schools have a little more playing room. From what I've heard, the lower third of a class at schools like Hopkins, Harvard, Duke, UCSF, etc.. still lands kick ass residency.