Coffee Lake Intel 8th Gen chips are arriving in 2H 2017, so I hope they delayed the iMac to have those processors put into the new iMac in October (hoping for October rather than December). The high-end desktops will appear somewhere in July-onward.
Let's hope they don't give us Kaby Lake with DDR4 at that time. I'd buy a couple desktops, if that is the case.
To be completely fair, the new iMacs could come at WWDC.
Also, Coffee Lake is going to use the same LGA1151 socket as both Skylake and Kaby Lake used. So it could be possible that they could launch with Kaby Lake iMacs in the summer at WWDC, and then update the computers with Coffee Lake when that launches in 2H 2017. (Assuming 200 and 100 series motherboard chipsets get a BIOS update to support the new CPUs like Kaby Lake).