You're assuming that a suit always provides a good impression. That's not automatic, especially if you're interviewing at a small, casual company that is trying to maintain a specific kind of casual, fun work culture.

Like, say...a startup.

Work for a startup. Second startup actually, both successful. Wore a suit to both interviews. Results paint a pretty clear picture.

Even companies looking to maintain a casual, fun work culture should appreciate their candidates attempting to present themselves in a professional way. If an interviewer casts someone in a negative light because they wore a suit... I'll go out on a limb and risk getting flamed by saying that this person probably shouldn't be responsible for hiring in the first place.

We haven't had many people show up in suits to interviews at Justin.tv, but it's a reasonably accurate indicator of failure. I try not to prejudge, but there's only so many times you can see a correlation and not start to stereotype.

I agree. I think a lot of the advice that I see above is: "come as you are, and if they won't take you, you didn't want to work there anyway."

This is wrong. I work for a startup (my third one). I wore a suit to the interview. My now-boss said to me, "We should have told you that you didn't have to wear a tie to the interview. Sorry!". I would have worn the tie anyway.

I have personally hired 16 people since starting work at my current startup. They all dressed professionally for the interview, and subsequently changed into shorts and t-shirts.

The issue is: while the startup is casual dress, I might need to take you with me to pitch Fidelity Investments for the B-round of funding. If I'm taking you with me to my investment bank, you better wear a suit and be comfortable in it.