Tab bar icon replacement
On the iPhone home screen:
There’s a trick to replacing the 4 persistent apps in the Dock at the bottom: you cannot drag into the Dock to bump them out of the way; instead you must drag something out of the Dock to make room first, and then you can drag an app into the free space
..in comparison to the black 'tab bar' navigation widgets with the 'edit' mode buttons, which you can't drag icons out of, but dropping new icons on overwrites whatever you dropped them on. Yay consistency.