I show up at interviews, and to meet about local contracts, with a dev t-shirt and jeans.
If they want me to dress a specific way, then they're obviously interested in a fashion model instead of a developer.
If I go to a design-oriented interview, I either wear t-shirts related to their product or t-shirts with ascii art. Sometimes, if I actually want to work with them, I'll literally design, print and wear an ascii art t-shirt of their brand or one with an illustration of a mascot/character from their product - bringing up to three t-shirts to give away.
It's fun to do that. And, again, if you're a developer and they verbally would have wanted you to be a fancy pants schmuck - then walk right out and let them get what they deserve. ;)