I've seen Skylake on Dell machines, hell I've even seen deals for Skylake motherboards and processors from the component shops.
Skylake is out there.
However AIUI these are desktop class chipsets and the iMac tends to use the laptop components for heat dissipation reasons.
I've not seen much in the way of Skylake laptops although most manufacturers have announced that they are on the way or they are just hitting shops now in serious quantities.
It's therefore pretty nailed on that Apple have the chips, possibly not enough for production purposes depending upon exactly which option they are interested in but they will certainly have them on development machines.
It's not beyond reason that if supplies of the correct chips are limited right now, they aren't far off being available on the kinds of scales required for mass production.
So there is absolutely nothing to stop Apple announcing Skylake iMacs next week even if they won't be available until the back end of the year.
That said the main issues for me aren't necessarily related to Skylake. I mean DDR4 would be nice and Target display is on my 'nice to have' list, but they aren't essential. TB3 and USB 3.1 are of more interest but to be honest they aren't deal breakers. For my data requirements TB2 is plenty fast enough and I'd prefer it to gain enough acceptance for the prices of TB hardware to fall more inline with USB.
What I really want is a GPU bump, better handling of the heat issues, 512gb SSD as standard and no change to the ability to upgrade the RAM at a later date.
If those can be met on the next RiMac then I'll pull the trigger
I agree with all of this, except that the retina iMac DOES use desktop-class CPUs, so Skylake SHOULD be possible. I will be severely disappointed if there's no 27" update, and/or no Skylake in it. The CPUs ARE available. Just the mobile ones that aren't