John Carey at 50 Foot Shadows:

All in all it seemed obvious to me that whoever at Apple was working on the effect found the ideal amount of give to the parallax panning to get a natural feel and set the dimensions of new desktop images to fit this ideal down to the pixel. Therefore, to get the most natural fit for your wallpaper images in accordance to their current programming I highly recommend you crop iPhone wallpapers to 744x1392px and iPad wallpapers to 2524x2524px.

So there you have it: the ideal resolutions for wallpapers that will appear properly at whatever rotation and with the right kind of parallax scrolling.

Speaking of, John has a fantastic collection of wallpapers available that you can purchase for both form factors combined for just $10. I recommend them because they are awesome.