Livefor32, I interviewed at CP for the past two years and saw a wide variety of things. The first year, I wore a really nice sweater and a nice black skirt. Last year, a nice blouse and a nice pair of slacks. This year, I'm wearing a nice pair of black dress pants with a white/cream colored jacket and black shirt underneath. I know that doesn't *sound* like a great combination, but the jacket looks really nice on me and I love it. My only problem was that last year, I got a bit hot in my blouse and I think a little bit sweaty. I don't think it was noticeable, but it still made me feel more nervous.
At my interviews, I saw a decent number of people dressed like me (nice shirt, dress pants), and a slightly larger number wearing suits. Even some of the female interviewers didn't wear a suit. (All the guys wore suits, of course.)
I think the best option is to wear something fairly conservative and understated (by which I mean everything's covered, no super bright colors, no low cut tops, etc), but beyond that to wear what you like. If you go into your interview feeling and looking horribly uncomfortable in your suit, you won't interview well. Likewise, if a sweater will make you feel horribly under dressed, then don't wear it. Just make sure you're comfortable and confident!