But maybe I'm misinformed ... Are you saying developers have been planning for a 16:9 retina display since the beginning, just in case Apple developed a wider aspect ratio, so all the existing apps will just suddenly fill up the entire screen?
Actually if Apple did anything, I would imagine they'd offer a way to wallpaper the black bars on apps that didn't fill the entire space. That would satisfy my mom. Nothing gets her riled-up like black bars surrounding her picture on her TV!
Open up Mail or Safari on the iPhone or Touch. Now rotate the orientation 90°. The width and height will adjust to fit the relative width and height. If the developers have standard app developers have done their coding properly their UI elements should adjust regardless of the resolution used.
The only real caveat I see are from game developers who obviously don't use the UI APIs in xCode. Those would then get the bars until those devs can adjust the app accordingly, if ever, but I doubt that will be too much of issue for our mothers. For example, I imagine WwF would have the bars but would probably be adjusted with more data of the game but the tiles would still be the same size.