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.