Phone covers. If functional, they will add some mm to the thickness of the phone. I believe overall smartphone thickness to be the sum of phone + cover.
This sort of cover could be integrated (via screws or mechanical latches that would be unsightly on a bare phone) with the phone skeleton, making for a package which is both stronger and sleeker than, for example, iPhone + OtterBox.
iPhone? Sleek, thin, $700. Better put a cover to protect this piece of art!
It drives crazy the engineer in me, but that's the way it is. And it's about as rational as buying a nice car and letting it sit in the garage.
Maybe instead of making thin phones so people can have covers, the focus should be on making more durable phones that don't need covers.
(Then again there's the whole obsession with personalization, especially in Asia - but they'll put rather bulky things on the phone in the first place, so thickness should not be a huge issue)