We touched on this already in our iPhone 4 review -- in iOS 4, Apple has set up several types of behavior that, when taken together, create the multitasking experience for the user. What the user sees is the fast app switcher that launches when you double click the home button, and the carousel animation that sends one app to the "background" and another to the "foreground".
For this to work in a user-friendly way, however, developers need to implement something called "saved state", so a snapshot of the app's current position is stored in memory, and when you return to it, restored to exactly that position in the app. This is supposed to eliminate what used to happen with 3rd party apps in previous versions of iOS -- when you left an App Store app and came back to it later, it relaunched from the beginning. This was especially frustrating, for example, if you were playing a game, got interrupted by a phone call, and then went back to your game only to have to start over.
Ideally it shouldn't happen any more on iOS 4 compatible apps. But it is. What some developers are presenting as updates, and Apple is listing as iOS 4 compatible apps, aren't implementing saved state.
While I'm not certain, it appears the baseline for iOS 4 compatibility is simply that the app isn't supposed to crash when launched in iOS 4.
And that's not enough.
Users need a consistent experience. iOS 4 supports multitasking. They download an "iOS 4 compatible app", use the fast app switcher to leave and come back, only to have the app re-launch and they think multitasking doesn't work or iOS 4 doesn't do it.
I watched, rather incredulously, that very thing happen on a TV review of iOS 4 a week ago. First I though the host should have (and probably did) know better. Then I realized the reviewer didn't matter -- users would probably experience the same thing and could justifiably come to the same conclusion.
Likewise, iPhone 4 adds Retina Display support to the top of the update list. Pushing out an iOS 4 update without an x2 icon at the very least is strange. (Pushing one out a first party app the day of iPhone 4 pre-orders without an x2 icon is bewildering -- looking at you Apple Store).
No doubt there's work involved and time required. I'm not a developer so I can't speak to how hard it is to implement saved state; I am a designer and I have some idea what's involved in making x2 artwork, but in both cases it obviously requires effort.
Some developers are clearly putting in that effort, however, and providing a wonderful iOS 4 multitasking and iPhone 4 Retina Display and even gyroscope implementations. Predictably they're the usual suspects like Real Racing and Twitterrific, and other apps where the developers clearly love iOS and really want to provide the best user experience possible.
Why isn't everyone?
(That's a serious question, if you're a developer and I've got it wrong, set me straight in comments or via email. If you're Apple, please consider not listing something as iOS 4 compatible or "awesome" unless it implements the awesomeness that is iOS 4.)