Version: (using KDE 4.0.0) Installed from: Ubuntu Packages OS: Linux Menu->View->Preview [X] : it seems more logical, for a global option like this, to be true for every folders.
In Dolphin the user can decide whether properties are valid per directory level or globally (see Settings -> Views -> General). It is not planned making each property (view, sort by, show hidden files, show preview, show in groups, ...) configurable by its own.