I think the PPI thing is somewhat misleading. PPI is a linear count, not an area. When you look at a display, it's a 2D image. We are not just looking at how sharp a single line is.
Many people think that the Retina (264 PPI) iPad is twice as sharp as the iPad 2 (132 PPI). In my mind, it's actually FOUR times sharper. Maybe it's harder for some to comprehend or market because he number gets much larger to compare, but here's a conversion to SQUARE inches (PPSQI...rolls off tongue, right?)
iPad 2 = 17.4K PPSQI
iPad Mini = 26.6K PPSQI
iPad Retina = 69.7K PPSQI
My main reason for pointing this out? I think the iPad Mini will look a bit sharper than people are expecting. Not just 23% sharper compared to iPad 2, but more like 53% sharper. Obviously the Retina iPad is still way sharper.