Back in Qt 4.4, QFileDialog was rewritten in order to help with performance issues. In addition to that a new model was designed to read the hard drive content: QFileSystemModel. The main advantage of QFileSystemModel is its thread: the model is populated asynchronously by a gatherer thread. The benefit is that the GUI is not stuck while the gatherer is reading the file system (except one case). In 4.4.x, 4.5.x, 4.6.x the focus was to mature QFileSystemModel to make it fast and stable. I think now it has reached this status and QDirModel can be easily replaced by QFileSystemModel as it provides the same features and more.
So in 4.7, QDirModel will be obsoleted and QFileSystemModel will be used in our examples. At the same time QCompleter will gain the support of QFileSystemModel. The commit is 319b0262418d74cc416a7dd1f620b54ba45bad22 and you can find it in our trunk.