It probably helps some...I mean if you've done a significant amount of research (i.e. more than a summer program) then you would expect to be on one. Being first author is better, as it says you probably did most of the work on the project. But I don't think it's going to give you a definite acceptance to the school of your choice. It's a perk, yes, but it's not going to support a mediocre application.
That being said, I hafta say one of my biggest pet peeve about premeds are the ones that go shopping for a lab to find the one most likely to give them a paper or absolutely EXPECT to be put on a paper and publish. If you like research and you're actually contributing to a lab and do something that's publication worthy...that's great! But if your plan is to find a lab and do research just to get a paper to put on your resume when really you're just a lab monkey...
As for MD/PhD applicants...a good number of them have done a lot of research, far more than the average med student so while it's a good thing, it's definitely not going to be unique.
