I have pretty strong feelings about this, so be warned I'm biased, but I'll give my $0.02.
I think pretty much anything other than 1st author publications as an undergrad should mean nothing more than the time and experience in the lab. I saw a girl get a 3rd author pub after just 2 months of mindlessly doing what a post-doc was telling her to do for 5 hours/week. The girl failed out of our program a month later. I've also seen people fight to work with a post-doc because she was absolutely brilliant and working under her for a summer meant 2-3 2nd author publications. Then you have students who slave away in a lab, produce a huge amount of data for a paper, and don't even get an acknowledgement because the PI of the lab doesn't believe in publishing undergrads (happened to me, hence my bias).
That being said, a first author pub is undeniable proof that you had to think through the experiments and plan/execute them successfully. However, because research publication opportunity is SO variable in undergrad, I think the biggest contributing factors should be the LOR and the student's knowledge of his/her research at the interview. If you've ever talked to an undergrad about their research, you know pretty quickly whether or not they are the mind behind the experiments or simply an extra pair of hands in the lab. Any student can regurgitate the contents of a review they read on their field, but someone who truly contributes to the research will know how each and every step of a reaction/process contributes to the final result, and in great detail.