I'd say the reason that Apple hasn't gone for more pixels per inch on their computers has to do with the lack of resolution independence in their operating syst

I'd say the reason that Apple hasn't gone for more pixels per inch on their computers has to do with the lack of resolution independence in their operating system (though it's been on the horizon for several years now). Making everything denser makes for harder-to-read menus and controls, unless the software (including third-party software!) is smart enough to compensate for it. With the iPhone, they're presumably addressing the software aspects of the resolution change in addition to just bumping the pixel count.