Resizing only means having to rewrite apps if the screen resolution changes -- especially if it changes by something other than a whole-number multiple (e.g. 1.5x versus 2x). All rumors indicate a 3.7-inch screen iPhone would have the same Retina-Display resolution (still maintaining over 300dpi).
Technically their "Retina-Display" stuff is based also on typical viewing distance as well -- so a "Retina Display" iPad, iMac, or MacBook (assuming those are in the works) may not go as high as 300dpi. However, a Retina-Display iPad would like require the same pixel-doubling (2x) that was done for apps not optimized for the Retina Display until updates came that included higher-resolution graphics.