Honestly, it depends on the interviewer. For just about all of them (myself included), why you're choosing Penn is a huge deal: you should have something pretty substantial to say about it that shows something unique about you and shows that you've done your homework about the school. Not an easy task, but I think it's something most people can do with a little thought and reading.
Other than that, it's up in the air. I'd like to think that most interviewers, at the end of the day, want to talk about what you want to talk about: things that you're passionate about and like to think about and do outside of school (and within) are always safe bets, and they're usually subjects that get people to open up and show (albeit briefly) what kind of student and person they are. I always like to ask what people are reading (outside of school) and what current events they're keeping up with.
@yoyo17: I didn't know Penn had that reputation. I'm not even sure if it's justified or not, but I'd still say play it safe and take your interviews seriously at all your schools: it's not really that hard to prepare for them, and most people can usually tell when someone hasn't.