Odds are, the iPhone 5S cases will be compatible with iPhone 5 cases and vice versa. There'll be a line of 5S cases only for the marketing, I would think. There are two things that wouldn't make this the case (pun not intended 😛):
The home button w/ the rumoured fingerprint scanner may cause some issues for iPhone 5 screen protectors and cases with lay-on-the-table design (doubtful about the latter, though).
A second issue with iPhone 5 cases is whether the camera cutout will fit the dual LED that's rumoured to in the 5S. Then again, I think current cutouts are large enough.