That might have been true in iOS 2.x. But in 3.x, and now very strongly in iOS 4.x, Apple's development guides all use a Point based rather than Pixel based system. Points are a unit of measure that is independent of Pixels. For example, a button might be defined as 40 points wide in software. On an iPhone 4 this will be 80 pixels. On an iPhone 3G, it's 40 pixels.
The ratio between Points and Pixels is called a ScaleFactor by the SDK. Scale Factor is a Float - it can be any number between 0.0 and well, some ungodly big number.
All of the major API's, Quartz, UIKit, and CoreAnimation are Points based. (not Pixel based)
If they roll out a screen that is 1200 pixels instead of 1024 pixels, they just change the ScaleFactor for that UIDevice to 1.2 and your 40 point wide button is displayed at 48 pixels wide. No big deal.
Developers already have 3 resolutions to deal with. Most of us are using scaling factors already to simplify support for Universal Apps.
Hope this explanation was helpful. If you want to read the public guidelines, you can visit:
http://developer.apple.com/library/...endence/SupportingResolutionIndependence.html
Skip down to the section heading "Points Versus Pixels". It's a nice fairly non-technical explanation.
- Don