I'm trying to make an app that displays something in real-world units. It's not a ruler app, but it wants that kind of precision.
It already looks like the iPhone and iPod touch have different screen resolutions (160 & 163 respectively)
I've found this Calculating pixel size on an iPhone and this iPhone screen resolution changes in future hardware and this http://forums.macrumors.com/showthread.php?t=350612
From my reading it sounds like I can treat the 320 * 480 screen space as 2 * 3.5 inches, ignoring the difference between the iPod and the iPhone resolution, which only seems to affect the clarity of the image, not the size.
My question(s) are: is this true? any way to determine this (320 "pixels" == 2 inches) in code, so if it changes I don't have to update.
I wouldn't worry about it for now, if Apple changed it and didn't tell their developers then they would lose customers themselves. Just hard code your app to work in this environment for now.
The PPI shouldn't be different, but all other apps run fine and the only hardware differences Apple tells us to account for are microphone, camera, Internet connection, and phone-ness. (Even if the PPI is different, it's still 320X480 pixels no matter how many inches are used.)