This is reported against 4.5-beta1. The correct interaction model for UI elements like checkboxes and buttons is as follows: After you press the left mouse button on them, you are supposed to be able to cancel that operation by dragging the mouse pointer away and releasing the button outside the UI element you originally pressed on. This release then has to be free of side-effects. A KCategoriedView with items with embedded checkboxes currently violates this model: If you press on one checkbox, then drag away and release on another, the checkbox you released on gets checked. A check has to be made to ensure that both the press event and the release event occured above the same checkbox before a checkbox gets checked. An example of where this is an issue is the "Display" page of the "System Tray Settings" configuration dialog of the System Tray plasmoid in KDE 4.5-beta1.
The bug is in Qt; the item is a checkable item, and the event handling bug also shows with a plain QListView with checkable items (see Dolphins "Services" settings page). Please report this bug to http://bugreports.qt.nokia.com/