If you where a little disappointed after the last WWDC 2012 event in which Apple is focused on laptops and played virtually no attention to its desktops, I’m afraid you’ll have to wait even more. Especially if you meant to buy an iMac equipped with the famous “Retina Display” screen, which has acquired so much success with the new home portable.
Rumors were already being talked about for days and seemed to be confirmed by the Instapaper developer, Marco Arment on his blog stating that after learning through diverse data sources within the company, everything suggests that the next iMac will be comming on autumn this year … but would not include the new Retina Display screen.

The reasons wielded to support his theory, indicates that such “retina” panels of 27 inches are also expensive to manufacture and production is slow, not having sufficiently constant flow of such panels to ensure the manufacture of equipment at the pace required. Moreover, given the size of the panel, it could have limited bandwidth with its Thunderbolt connection, so they need an update for maximum speed supported by the end of 2013 for its Thunderbolt connection to support “iMac Retinal Display”.
So, I think we still have a year to wait to get hold of an iMac with the screen so much desired throughout the world.