One of the few remaining complaints against Qt applications using QGtkStyle on GNOME have been about the file dialog. So far it certainly looked like a GTK+ dialog, but it was not exactly the same dialog that other applications had been using. While I'm not really in a position to comment on which dialog is the better one, this is what it currently looks like with QGtkStyle:
However, the policy on other platforms such as Windows and Mac OS X has been to use the native dialogs if possible. I recently applied some patches to support this for GTK+ as well. This is what you will see if you open forms in Designer now:
Note that since the KDE file dialog provides somewhat different functionality from the Qt one, KDE applications will still use the KDE file dialog. Feel free to try it out directly from the svn on my labs page and report whatever issues you might find here.