I highly dought the iPad will get a retina display in it's next iteration, because if it did, it would have the largest number of pixels in any apple product. Including the 30" cinema display and the MacBook pro 17". The cost would probably be astronomical.
Please, give it a rest, this is never going to happen.
Cost isn't the only issue, the battery power and processing requirements to run such a display would be orders of magnitude greater than the current model. Then you need to take into consideration overheating concerns since the device is so small and encased in aluminum.
I suspect they will increase the resolution in around two years, but it won't be to the same degree as iPhone 4 vs. 3GS. I'd say 1600 x 1200 max in it's current size.
That said, I'm perfectly happy with the current resolution. Too many people get caught up in specs that don't really affect the usability of the device at all. Yes, the iPhone 4 display is the sharpest display Apple produces. But using it is pretty much the same as using a 3GS. You'll still need to do a lot of zooming in and out in Safari, and every other app functions the same. The same would be true of a higher res iPad.