Business casual is generally the way to go - so a shirt, optional tie, and dress pants. Remember while they want you to look reasonably smart inside a courtroom they also want you to be comfortable - if you're sitting there uncomfy because you're not used to what you're wearing then you're not giving your full attention to the case. The letter you got calling you to jury duty should give advice on what to wear, or at least a phone number for the court you can call and say "Hey, this is what I am planning to wear for jury selection. Is it OK?" and they'll either tell you yes or give you advice on what you should be wearing instead.
Of course this is all assuming you *want* to be selected for jury duty. If that's not the case then turn up wearing a t-shirt with a political slogan on it & carrying a copy of The Communist Manifesto or Mein Kampf.
Oh, and I almost forgot - if you do get selected it's a good idea to have a light jacket you can take off/wear inside the courtroom depending on how hot/cold it is.