Speculations are mounting that "Apple update the iMac to include iOS within the next 60 days." Word is that it'll run a traditional OS X install, and will also allow users to activate "touch screen mode," launching them into iOS. The iMac should be equipped with both Mac OS X and a touch interface for iOS.
Steve Jobs was recently quoted saying that Apple has no plans to introduce an App Store for OS X, and with iOS on the desktop, they mayn't need to.
There're also reasons for which this rumor seems rather suspect. Capacitive touchscreens are commonplace in the mobile sector, but producing capacitive touchscreens at 20+ inch sizes is something that has yet to be seen. The economics of producing such large versions mayn't yet be feasible.