Lukas Mathis has been writing some of the most insightful essays on UI design I’ve read in ages. He has a great piece today on modes and “quasi-modes”:
Quasimodes require the user to do several things at the same time, such as holding down the Shift key while typing. Modes, on the other hand, allow users to do things sequentially — hit Caps Lock, type, hit Caps Lock again. Sequential actions, especially if guided well, are often easier to execute than parallel actions.
And he argues (correctly, I say) that the iPhone’s new modal interface for selecting text is superior to the WebOS’s quasi-modal interface.