Despite the fact that the Coffee Lake processors stick with LGA1151, they are not compatible with current Z270 or older Z170 based motherboards. Hardware.info already got a sample of the Core i7-8700K and a Z370 motherboard and decided to do some testing.
The site confirms Coffee Lake doesn't work in Z270 motherboards and another interesting conclusion is that there appears to be no backwards compatibility in the other direction either. Hardware.info was able to test just one motherboard but a Kaby Lake CPU refused to boot on this Z370 board.
The site also uploaded a photo that compares the socket of the Intel Core i7-7700K (left) and Core i7-8700K (right) CPUs.