As was said, AOA is offered at certain schools (though I was under the impression it was the majority of schools). The qualifications vary by school, but generally 10% of any given class is given AOA distinction. It is not simply handed out to the top 10% grades-wise. At some schools, you have to be in the top 25% grades-wise and then they start looking at your rotations, your EC's, etc... before deciding on the 10% that will be given AOA. In other words, you could be #1 in the class grades-wise, but theoretically, you might not get AOA (though that's unlikely).
I thought getting AOA was pretty important for getting into top residencies, but maybe not since some schools don't have it. Then again, it could just mean you were near the top of your class anyway which generally is necessary to get into a top residency.