Not all app in Apple's growing "awesome iOS 4 apps" list are "awesome" -- at least not in their implementation of iOS 4 or for iPhone 4.
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.