I seriously doubt Apple will release a display at any iOS-related event.
It seems more likely to me that they would announce such a mythical display alongside mythical higher-resolution iMacs, probably about the same time that Yosemite is actually ready.
Or not. They might just make a silent update. Although a 4K display warrants at least a mention from Apple on-stage.
By today's standards the current display is really quite sad. When it came out, 27" / 2560x1440 / $999 was a pretty good deal. Has Apple even updated the thing to match the styling of the new iMac?
I **really** want Apple to release a 4K display. I have one of their old 30" displays, and while it isn't dying, it's time to replace it. If they come out with a 27" 4K, I'll buy two immediately. It's probably too much to hope that they'd come out with a 32" >4K; they have to get the panels from somewhere.