U.S. Cellular announced earlier this year that they signed an agreement with Apple to carry the iPhone. They will be carrying the 5s, 5c, and 4s. They have not given any specific launch date yet. All they have said previously was "by the end of the year" and have lately started saying "very soon". I think they are waiting until their LTE expansion has progressed a little more.
I just went to a local U.S. Cellular store yesterday, and I was told that they think the iPhone will be available mid-October. Of course, this was not an official answer. I sure hope it is October 1 instead, because I am a U.S. Cellular customer!