For anyone thinking Apple will release a new iPad in the fall... don't bet on it. The last two releases have shown that it takes AT LEAST several months before supply is able to adequately meet demand--especially when you look at it from a worldwide perspective. Do you really think they'd risk a shortage during the holiday season? Not a chance.
What if the 3 was pitched as a new tier, rather than a replacement? A retina class screen would probably still squash the margins to levels unacceptable to Apple. If they could simply sell the device for a higher price point, or slightly drop the iPad2s price point, then there's no real shortage risk.