In Arizona, there is talk about making every voter prove that they are a citizen by showing a valid ID before voting. The only thing holding this from being a passed law is the federal law that says that voters may vote if they mail in a certificate proving they are US citizens, and then swearing they are before they vote.
I think that it is interesting that people can simply swear they are US citizens before voting rather than show identification. I also think that it is a good idea to have to show a valid ID before one can vote, and I think that should be a federal law. With how many identity thefts that occur daily, you would think all things would require a valid ID.