I definitely agree with the other responses that it is better to be potentially overdressed. Our firm has a very relaxed in-office dress code. However, if you are headed to meetings, you are expected to dress a little nicer. If the company is interviewing for a project, you are expected to dress closer to business professional. Most interview candidates seeking employment dress somewhere between business casual and business professional.
Unless the firm you are applying with makes people wear jackets and ties, as a general rule I think you should go to an interview dressed up more than would be required for a normal business day. You want your appearance to show that you are taking the interview seriously.
Specifically for ladies:
You shouldn't wear high heels unless you wear them regularly enough to walk in them confidently. The way you enter a room says something about you, and you don't want that to be overshadowed by seeming unsteady.
------------------------------
Heidi Wallace EI, A.M.ASCE
Engineer Intern
Tulsa OK
------------------------------