The company I work for has a very casual dress code. Jeans, sandals, sun dresses (with a minimum length). The only thing that they absolutely do not allow are things like track pants and what can be considered promiscuous attire.
I can't speak for others, I personally enjoy wearing everyday clothes to the office. My job does not have me interacting with customers or clients in person and I appreciate being able to dress casually. Would I feel differently if I did have to interact with people? Yes, I would feel differently, the clothes you choose to wear will always give the person you are meeting a clearer picture of the type of person you are before you even open your mouth.
When I interviewed for my current position I did not go into the interview wearing jeans and sneakers, even though I knew that the dress code was casual. I went in there dressed professionally and ensured that everything I was wearing was neat and tidy. Would going in with a more casual outfit have made me a less likely candidate? Could a more casual outfit take away the opportunity? I don't believe so, but I do believe that going to the interview dressed professionally gave me a slight edge in showing the interviewer that I meant business.
There are quite a few articles about the do's and don'ts, the one thing I will always adhere to will be to always appear presentable and never like you just rolled out of bed and threw something on.
Below is a great link that has a list of 12 guidelines.
http://theundercoverrecruiter.com/how-dress-your-job-interview/
