Retina displays have 'double pixel density'. (The actual PPI varies between screens anyway, so this is a nice simpler way of differentiating between retina style displays and normal ones). For web stuff anything measured in 'pixels' is sometimes referred to 'css pixels', as it doesn't necessarily correlate to screen's pixels. A button 100px wide will be drawn with 100 pixels on a normal monitor, but 200 pixels on a retina/DD display.
So you can just half the phone's resolution and scale up at runtime if you want to retain a pixel look, or just embrace the HD size, and have extra detailed sprites twice the size you were expecting them to be.
Photoshop, as far as I can tell, doesn't do anything with PPI info outside of print, except maybe how it calculates font sizes.