Seems a bit stupid now the iPhone 4 has been out for so long, but there we go.
I don't think it's stupid because Apple knows people will be lining up by the hundreds on release day to get one. It is a great way to extend product life a few more months without dumping a lot of cash into a re-designed phone. By doing this they can get 15-18 months out of the product cycle instead of just one year.