There have been rumors for about a month that the iPhone 5S home button is going to be ringed in silver. Clayton Morris even told This Week In Tech that he was "unequivocally" sure about the change, and that the home button would otherwise have the same concave design. Other leaked photos have supported the rumor, though as usual their authenticity is somewhat questionable.
Perhaps the most solid evidence comes from the graphic Apple included with their media event invitation for September 10th's announcements (above). In addition to colorful circles that are thought to represent the colored plastic casing of the iPhone 5C, the empty silver rings could allude to a ring around the home button on the 5S.