There is much debate, while there will be new Polaris graphics cards available, it's looking unlikely that Kabylake will be with us in the form that iMacs need until Feb/March next year.
So Apple have a couple of choices do a silent update and put in the new graphics with the current skylake or wait for Kabylake next year and do it all together.
Sorry but which way they are leaning is completely unknown at the moment, but looking at the wait on chips etc for the rMBP's and how long that has taken and delayed new MacBook pros means it could be more like spring next year.