The iMac could have been upgraded anytime in May, because the new desktop processors and the video cards are already here, but nothing happened. And the only rumors we see are about a refreshed MacBook Pro, the one which was updated more recently. What will happen to the other Macs?
Here is my take on that: The MacBook Pro (13" especially) is Apple's cash cow in computers, followed by the Air. The iMac is far behind in sales, and the refresh will be less interesting (who cares if their iMac weighs a couple of pounds less or is 1/2 inch thinner). So they wouldn't want to take away from the big news of the MacBook Pro changes by refreshing the iMac first.