Thomas Verschoren musing about a future Retina iPad mini if it used the same 326dpi display as the Retina iPhones:
The downside: the iPad mini with a 326dpi display have a far better retina quality than the iPad at 264dpi. Selling the retina iPad as retina wouldn’t really be justifiable I think.
I don’t really think this would be a deal-breaker. It looks like iPad mini is going to become the de facto iPad, and so if Apple can sell more of those than the bigger iPad, I’m guessing they will. Why not make your most popular iPad also the best iPad?