I think it’s as much of a certainty as Apple announcing their next iPhone. I mean, thanks to Gizmodo and the umpteen billion leaks that have come after, we know most of the specifics about the iPhone.
Could Apple make a splash by announcing it’s going on all four U.S. carriers? Sure, but again, that wouldn’t be a surprise but, rather, more of a confirmation.
I think a ChromeOS release would upstage an announcement that, frankly, has had a bit of life sucked out of it.