When the iPad was released, things were simple: iPhones had 3.5 inch screens, iPad had 9.7 inch screens. Apps were designed either for iPhone or for iPad (‘Universal’ apps just basically packaged the two UIs together). This certainly helped the iPad succeed: designers were forced to create a specific UI for each device, and thus the iPad apps were really adapted to the big screen.
Now things have changed. iPhone sizes are 3.5, 4, 4.7, 5.5 inches, iPad sizes are 7.9 and 9.7 inches. There is almost a continuum of sizes, and designing specific UIs is not so obvious any more.
When I started developing OneReader, I designed quite different interfaces for iPhone and iPad (more buttons on iPad, more gestures on iPhone…). The iPad interface scales well to the iPad Mini, and the iPhone interface works fine on the iPhone 6. But none is really suited to the iPhone 6 Plus. I guess it is the same for many apps — you cannot just design the same way for 4.7 and 5.5 inch screens.
In the meantime, Apple has provided developers with new tools to cope with different screen sizes: Autolayout since iOS 6, and now size classes among others. To app developers, devices do not appear any more as iPhones or iPads, but as devices with regular or compact width, regular or compact height. The mapping to devices is not really straightforward: regular height and regular width is iPad, what is compact height and regular width?
Apple is really encouraging developers to stop thinking in terms of iPhone and iPad. And this corresponds to an evolution of usage too: big screen phones can do many of the tasks that were thought to be for iPads only.
I think what happened to Marco Arment and his accidental iPad version of Overcast may well just a switch flipped too early. I would not be surprised that soon all apps will be ‘Universal’ and required to make use of size classes to adapt. I would certainly design a new app now with an adaptive user interface for all iOS devices instead of separate iPhone and iPad UIs.
There are persistent rumors of a bigger screen iPad, and of the future possibility to split the iPad screen between several applications. Soon an app may be running on an iPad in an iPhone-sized window. For this to work, apps will have to be able to adapt, and not have two separate iPhone and iPad UIs.
With iOS 8, all the pieces are there to drop the distinction. Better get our apps ready for when Apple makes it happen!