Opinions on picking future teammates' brains over lunch?

I have a big interview scheduled and lots of time to prepare for it. I'd like to know what I should be focusing on in that time - that is, what competencies this company in particular values, including the ones that I wouldn't have thought of just by reading their website. So it occurs to me that the best place to go is the brains of the people who are already there. If I took them out to lunch, got friendly with them, found out exactly what a Company X employee is expected to be able to do, I'd have a big leg up. This also shows that I care about the opening and am willing to allot some of my own time for it, and serves as good PR.

Or does it? I don't want to be viewed as some kind of ingratiating snake, trading sushi for favors. This is worrisome enough on its own, but I've already been informed that the nature of the interview will be heavy on earning the impressing my future teammates. How can I communicate that I'm interested and gregarious but not devious or compensating for lack of skill?

This is a Bay Area startup made mainly or even entirely of young geniuses, for what the demographic matters. Treating an employed programmer to lunch and picking his/her brain is something I've been meaning to do for a long time outside the context of learning about a specific company; I've just never, you know, DONE it. So what do you think? Good idea, or going too far?

Oh, and what are YOUR tricks?