This is why, if Apple quadruple iPad resolution, it can't named iPad display "Retina" --> (translated in english from italian) http://bit.ly/fnDNXB
Depends on the difference in distance people use an iPad at versus an iPhone.
iPhone - 10 to 18 inches.
iPad - 18 to 24 inches -> can achieve "Retina" with a lower PPI. But as I write above, the extra resolution required for this even would put a real strain on the CPU/GPU in a tablet device.