What? I've been an Android developer for years and never had to do this. I'm pretty sure it will be the same way on iOS as it is on Android - provide another layout (a different xib in this case) for the larger and smaller screens. Done.
The point here is that you have to create a new layout for the iPhone 5 (if you don't want your app letterboxed) and this adds an extra layer of complexity to the development. Seeing that there are also rumors about a different form factor iPad you would then have to maintain 6 different layouts (standard/retina/16:9 for iPhone, standard/retina/new layout iPad). Well or 12 if you have specific landscape layouts.
Designing UI on Android apps can be a pain since you don't have the "pixel perfect" paradigm, which is a common criticism on Android from iPhone developers, but if you manage to get them right you can get away with a single xml layout (of course, you can able to make things more complicated).
It seems to me that Apple are getting themselves in a bit of a mess with this. The tradeoffs with the "pixel perfect" method haven't really materialized until now.
Maybe I'm just being over pessimistic with this and it will be a non-issue, I really can't tell at this point, but I see this as the greatest issue with the iPhone 5 (which in turn is probably the reason they didn't increase the screen size earlier).
Just as developers slowly phase out support for older iOS versions (and by extension older devices), within a year or two it'll be possible to maintain only two layouts for the iPhone (retina 3.5" and 4"), two for the iPad (standard, retina) and one for the presumed iPad mini.
In time they'll be able to abandon both the 3.5" retina and standard iPad layouts and revert back to just one iPhone layout, one iPad layout and one iPad mini layout (assuming that's a real product) as the older models become obsolete.
It's not ideal, but it's still a relatively simple set of layouts that allow the 'pixel-perfect' method to continue working well.
Now, many apps didn't handle this, but this is really "more of stuff you should have been doing", not really "tons more work for iOS devs.