According to the docs, the differences:

If you are writing code that runs on both 2.x and 3.0 that needs to get tricky with tabs, these changes are a nuisance.

Even more infuriating is that the Apple documentation isn’t complete. The section ends with the sentence fragment “If you are implementing….” Yes? If I am implementing what? And what do I do if I am?

Yes, I’ve already reported documentation error. No response though. My guess is they’ll just remove the sentence fragment instead of expanding on it :(

[edited: I made a mistake in the table the first time I posted this. Should be fixed now.]

When the user switches tabs on a UITabBarController in iPhone OS 2.x, the sequence of events is:

1. UIView of new tab gets added as a subview
2. UIView of old tab gets removed
3. The delegate’s tabBarController:didSelectViewController: method is called

Run the same code on iPhone OS 3.0, however, and the sequence is this:

1. UIView of new tab gets added as a subview
2. The delegate’s tabBarController:didSelectViewController: method is called
3. UIView of old tab gets removed (sometime later, I think it’s invoked via one of NSObject‘s performSelector: methods)

This is a inconsequential change for most, but it might make a difference if you are checking the UIViews’ superview variables in your tab bar controller delegate method.