Uh, ppi scales linearly with pixel size; it's "pixels per inch", not "pixels per square inch" or anything of the sort.
So if existing iPhones are 163ppi and this is 2x as many pixels on each axis in the same space, that would be ~326ppi, not 652.
[EDITED to fix off-by-one errors in the resolutions]
The iPhone 2G–3GS all have a 163 ppi pixel density, while the iPad has only 132 ppi. Doubling each dimension of the iPhone's 320x480 pixels would result in four times the pixel information across the same area—making for a massive 652 ppi, nearly five times the density of the iPad, and well over twice what is generally considered sufficient for print resolution.
Much more likely is a 480x720 display, which would yield about 360 ppi.