There have been rumours 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 rumour, 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 colourful circles that are thought to represent the coloured plastic casing of the iPhone 5C, the empty silver rings could allude to a ring around the home button on the 5S.
Speculation is leaning toward the ring on the 5S only because of the rumoured fingerprint scanner and the form factor similarities (aka twin moment) between the iPhone 5 and 5S. It would make sense to highlight the fingerprint scanner visually and would also allow 5S owners to feel like they can strut their stuff with a phone that is recognisable as the new model. [Apple Insider]