Just had a play with the new iPad and iBooks.
When outputting a panorama you can use cube face size 2048, but the window and preview image size will still be 1024 x 768.
Same for when building a skin, the logical screen size is still 1024 x 768 although the actual screen resolution is 2048 x 1536px
So all your old iPad skins will still work

In the Mobile tab you can add 2048 as a level, I also set the cube face size to 2048, using the web server it loaded and displayed

Just for a test, my input image was 10,000px x 5,000px so I set the cube face size to 2,500px The iPad also displayed this


I also took it to 95% image compression and it still loaded. The images folder with three SVG buttons was 13.6 MB.
Regards,
Hopki