1080p screen for sure. Why? If Apple wants to keep the same aspect ratio and not produce blackbars for previous apps, or even worse, stretch them out, 1080p is the only way to go with a larger screen size.
iPhone, 3G & 3GS -- 480 x 320 iPhone 4 & 4S -- 960 x 640(Retina Display) New iPhone -- 1920 x 1080(HD Display, like the New iPad)
Notice the natural arithmetical progression in screen density. I really think we can expect this, folks.