How many apps depend on the absolute distance not changing as opposed to the relative distance not changing? I've never seen a ruler app.
As long as Apple doesn't change the ratio of pixels (480:320) and doesn't change the physical screen shape ratio (2.5:4" or whatever the 3.5" diagonal screen currently is, height and width wise), developers are fine.
The problem with Android is that the screen shape changes on different devices. That's the problem.