I know that, in the past, it has been far too problematic to design and market dust covers for computers. But with the release of the current-generation, DVD-less iMac in the Fall of 2012, alongside the existing Apple Thunderbolt Display, there should be few if any worries about changes in external size and shape of these machines. Basically, a 27-inch current-generation iMac is thinner than the Thunderbolt Display. So the design evolution is clear: They keep getting thinner and more like laptops.
I would really like to see Tom Bihn make and sell dust covers for the 21.5-inch and 27-inch iMacs and the 27-inch Thunderbolt Displays.