Looks like first Intel Coffeelake-S samples are already reaching reviewers.

Intel Z370 says no to Kabylake

Guys over at Hardware.info tested what we all wanted to know, the inter-compatibility between the sockets. The short story is that you cannot upgrade your Kabylake processor to a new motherboard (Z370), despite the fact that the motherboard has the same socket. It works both ways, because you cannot use i7-8700K CPU with the Z270/170 motherboard either, but that may just need a new BIOS. The new (Z370) motherboard should, however, already support Kabylake processors, unless, Intel had no plans for it.

A picture of Intel Core i7-8700K and 7700K was also provided. The new Coffeelake processor can be seen on the right side.

Source: Hardware.Info