Toshiba has a 720p "super" retina display at 367 PPI. It's going on to the Market this year. This display makes a four inch 1440 x 960 display even closer to existence. The four inch display said above would contain a ungodly PPI: 432.67. Developers wouldn't need to worry about size; its three times of that of the original, allowing for resolution independence "@3x".
Super Retina Display Source: http://www.ditii.com/2011/05/18/tos...-super-retina-display-coming-this-year-video/
Super Retina Display Source: http://www.ditii.com/2011/05/18/tos...-super-retina-display-coming-this-year-video/