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Resolution only matters when you are trying to match the size of an image across multiple devices (print, screen, etc)
App icons are always 57x57 and will display without any scaling thus resolution doesn't matter (you could save it as 72dpi, 65535dpi or missing the dpi metadata entirely--SpringBoard won't care and will draw it the same in all three cases)
Here is the link to Apple guidelines, with the sizes for iPad, iPhone retina, etc:
The "resolution" matters, but don't confuse "resolution" with dpi. Dpi is a measure used for printing, it means "dots per inch". It works like this: if you have an image of 100 x 100 saved at 100dpi, it will measure on paper exactly 1 inch (100 pixels per inch).
The dpi for the icons doesn't really matter, as they are intended for screen display only, not for printing.
It is customary to save images as 72dpi - the so called "screen resolution", because the old 14'' monitors (remember those?) could only display a maximum of 72 pixels per every inch of screen. This is no longer true, especially for the "retina" display of the iPhone. It has a much denser screen resolution, it can show much more pixels in a single inch than the older models of the iPhone. This is why you have to save 2 icons: one for retina display at 114 x 114 pixels, and another for the older iPhones, at 57 x 57. Again, the dpi doesn't matter, because whatever dpi you choose, the number of pixels on your image does not change: it would always have exactly 12,996 pixels (114 x 144).
I have had the best luck submitting to the App Store with 72dip, 57x57 icons.
The iPhone 3GS and prior display is 163dpi, so if you intend to create icons that are of a particular size relative to how they are shown, that should give you enough information. Note that the icon must be 57x57 pixels to be shown at that dpi.
The iPhone 4 and later and the fourth generation iPod touch and later have 326 dpi displays, and the icon must be 114x114 dpi to be shown at that dpi.