We have been working hard on improving our desktop integration with Qt lately. We automatically use the Plastique style if KDE is detected on startup. Starting with Qt 4.2, we intend to pick up the icon theme as well. This means that most of our standard dialogs, such as message boxes and file dialogs will use the same icons as KDE.
GNOME users will get the same treatment. The second image shows a message box on Ubuntu, using the Cleanlooks style and picking up the Human icon theme.