Both GNOME and KDE has a nice little control panel that allows you to configure how you want your toolbars to appear. Pure Qt applications show only the icons by default. Unfortunately, I don't think it is a good idea to suddenly change this behavior, but we can do the next best thing and allow you to opt-in. Starting with 4.6, you can use setToolButtonStyle(Qt::ToolButtonFollowStyle) on your mainwindow, tool bar or tool button to make your application dynamically follow desktop settings.
Here is a Qt app on GNOME with "Text below Items":
And this is how it looks with with "Text beside Items":
There are a few things you should do before you enable this setting though. First you should allways set QAction::IconText, otherwise your annoyingly long tool tip text will suddenly appear next to your icon. However, as you can barely see in the second screenshot, doing that is not enough to properly support Qt::TextBesideIcon since putting all that text into the toolbar generally takes up a bit too much space.
To solve this problem we added QAction::setPriority(QAction::Priority). In other words, you can now mark actions as more or less important. Quite handy for dynamic layouts determining which items to collapse first. In this case, actions with low priority will hide their label automatically when changing to this desktop setting.