You re excited to land that job interview. Now the question becomes what should I wear? Remember you only have one chance to make a first impression. Proponents of wearing a suit believe the suit must be worn for all interviews. First impressions are crucial and it shows the potential employer that you care. That you would rather be overdressed than under dressed is a strong argument for wearing the suit. It makes you feel better about yourself and more professional as well as adding an element of power. A lot of the more traditional companies still adhere to old school practices of a daily suit.
On the other hand a lot of the new age companies have a relaxed and more casual work atmosphere. Wearing a suit can be frowned upon and the company can look at you as somehow not "fitting in." Companies like Facebook, Google, Groupon are just a few of the many growing companies that have created a fun, hipper atmosphere for a place to work. A suit would never be seen in the office. You want to be comfortable with your clothing and it will be relaxing wearing casual clothes. Especially if the heat index is over 90 degrees you would not want to be in a suit. The pressure of an interview is stressful enough that you don't want to be sweating before the interview.
Researching in detail the company you are interviewing is a good way to gauge whether or not a suit would be necessary. You can always discuss with the recruiter or the human resource person for guidance. Whatever you decide, one thing is certain: make sure your clothes are ironed. Good luck with your job search!