It has pretty much been confirmed that Coffee Lake/3 series motherboards use a different version of Socket 1151 (keyed differently) so it is not physically/electrically compatible with Skylake/Kaby Lake chips. It will probably be referred informally as Socket1151v2.
Intel did something similar with Socket 2011 during launch of Haswell-E chips/X99 platform. They changed the socket around slightly which made it no longer able to work with earlier Socket 2011 chips and vice versa. It became known as Socket2011v3.
Ivy Bridge i5-3570K@4.0Ghz, Gigabyte Z77X-UD3H, 2x4GiB of PC3-12800, Sapphire RX Vega 64, Corsair CX-600 and Fractal Refined R4 (W). Kentsfield Q6600@3Ghz, HD 4850 2x2GiB PC2-6400, Gigabyte EP45-DS4P, OCZ Modstream 700W, and PC-7B.