I interviewed at Loopt (sama's YC company) 4½ years ago. When I went out to lunch with their tech guys, they joked about a guy who had come in before me in a suit, and how that really hurt his chances to get the job. Though I knew enough to dress casually, I was coming from a company in the midwest where I had to wear a suit every day. Both extremes seemed absurd to me.

I eventually turned them down in favour of another offer.