Following the example at http://doc.qt.nokia.com/qt-maemo-4.6/qmaemo5valuebutton.html I created a menu with the PickSelectorAction.

When the screen is in portrait mode clicking on the "selector" button to change a selection will force the application into landscape mode to show the list of options. I believe it is the same issue, or at least related to, dialogs that always start on landscape mode (regardless of the parent window orientation).