Someone sent me an e-mail asking what happened to the freedesktop icons I blogged about back in february so I realized I should probably write a followup. I'm happy to say that it has been merged into Qt 4.6 for some time already. If you look up the documentation for QIcon, you will find a new function QIcon::fromTheme(...) which essentially does what QtIconLoader::icon(...) did. Meaning Qt applications can now make use of the icon themes that KDE and GNOME ship with and use by default.
Since freedesktop icon themes only come as standard on X11 based desktops, you cannot use themed icons on other platforms such as Windows and Mac directly. This will simply return the fallback icon you provide. Allowing this would have forced us to decide on a standard icon theme as well as bloat the application by an extra few megabytes that many applications do not need. Instead we decided to leave this choice entirely up to the developer by making it possible to bundle themes such as Oxygen or Tango as part of the application resources and pointing to it using QIcon::setThemeName(...) instead.
This should make those pure Qt applications look a lot more integrated into the X11 desktop! Among other things, our trusty old Assistant client was modified to use the theme icons:
Screenshot showing of Assistant running on GNOME in 4.6.