Last night I was working through some issues related to updating older apps for the new iPad's Retina display. Now that there are essentially four screen sizes between the various iOS devices (not to mention the Mac), it's a lot more work to make sure images and icons look good. It'd be nice if it were possible to simply create one large vector graphic of each image and simply export it at all the necessary sizes, but this doesn't work well, especially at smaller icons sizes. The reasons are outlined in this great post about the dream of vector icons.
Some decisions I made in the past related to various icon packs (available in fixed sizes) and my own app icon designs make getting Retina-ready quite a feat. I'm trying to codify some practices to make this easier in case an iPad mini debuts a year from now.