DPI is a pointless term in comparing different medium. One of the comments mentions Dots per Radian of viewing angle which I think makes much more sense. Don't need/want my TV to have 300DPI, but 30 DPI would actually be pretty awesome there.
Of course, if there are enough dots in particular physical area, the human eye is incapable to see a single dot from the closest possible focusing distance. This is what I would call a "retinal diplay". Not sure if 326 DPI is enough for that, but is surely enough for the iPhone's screen.
[1] http://www.w3.org/TR/2008/REC-CSS2-20080411/syndata.html#len...