I just had the interview yesterday and they said they'd get back to me in about a week, I just wanted to type this up while it was still fresh in my memory.
I applied online a few months ago, never heard anything, applied again two weeks ago, read online that Petco keeps their job postings up year-round, gave up hope, then was contacted for an interview. They called while I was training for another job, and I got so nervous about how long it was taking me to call back that I let more and more time go by before finally calling back. Eventually I did and the person didn't even bring it up, just asked if I was still available for the interview time.
I showed up about 10 minutes early, met with the store manager and general manager and had an interview with the two of them, mostly just the store manager spoke the whole time. They were very friendly, mostly just asked questions about my pets and asked a couple work-related questions (they didn't ask me any "what would you do in this situation" questions, which surprised me). I've only worked at one job before, and that was just a one month seasonal position, so I didn't have much to say.
I thought it went well, but they had some other interviews to do first before they could tell me if I was hired or not. They said it would probably be about a week before they contacted me to tell me.
I wore black dress pants (that I bought from Macy's, to work at Macy's), a plain white long-sleeved shirt (not collared - I think it was a henley-style shirt), and a green knit tunic-type thing to go over it. Just dress nicely, don't worry about it.