I'd definitely like something a good deal higher than the current 1024x768, but as I'm a frequent portrait user for e-reading, I certainly want the aspect ratio maintained.
I'd love to see it happen, but I think doubled resolution (like what happened with the Iphone 4) is unrealistic for a couple reasons:
1. those displays can't be cheap to manufacture - the retina has one of the highest pixel densities of any display out there, and it's only affordable because of it's size. Consider that IPS displays capable of 2048x1536 res of any size run $600-700 at an absolute minimum (though different AR), and that baking those specs into a display that's only 10 inches is probably going only going to increase cost. I just can't see it being affordable enough to put in a mainstream tablet that's going to see a LOT of downward price pressure from emerging Android tablets in the next year or so.
2. Dramatically increasing resolution like that requires a commensurate increase in processing power and energy storage capacity. Even modern $300 video cards that draw 100 watts or more have some difficulty running games at comparable resolutions. Granted, 3d apps on the ipad3 probably won't be as graphically demanding, but suffice to say that hardware would need a serious shot in the arm to support that display.
I think a more modest increase to the standard 1280 res might be more realistic unless Apple is planning on bifurcating their tablet market into a "premium" market and a standard market. Naturally, that wouldn't make much sense either.
Just my 2c