Version:            (using KDE 4.4.2)
OS:                Linux
Installed from:    openSUSE RPMs

At ~kdebase-workspace-4.4.2/kcontrol/randr/cpp +122 it says:
        KTimerDialog acceptDialog(15000, KTimerDialog::CountDown,
                                   0, "mainKTimerDialog", true,
                                   i18n("Confirm Display Setting Change"),
                                   KTimerDialog::Ok|KTimerDialog::Cancel,
                                   KTimerDialog::Cancel);
At ~kdebase-workspace-4.4.2/kcontrol/randr/ktimerdialog.h +67 it says:
    /**
      * Constructor for the standard mode where you must specify the main
      * widget with @ref setMainWidget() . See @see KDialog for further details.
      *
      * For the rest of the arguments, See @see KDialog .
      */
     explicit KTimerDialog( int msec, TimerStyle style=CountDown, QWidget *parent=0,
                            const char *name=0, bool modal=true,
                            const QString &caption=QString(),
                            int buttonMask=Ok|Apply|Cancel, ButtonCode defaultButton=Ok,
                            bool separator=false,
                            const KGuiItem &user1=KGuiItem(),
                            const KGuiItem &user2=KGuiItem(),
                            const KGuiItem &user3=KGuiItem() );
So that looks correct (it also confirms that cancel is the desired default). My guess is KTimerDialog is designed to default to cancel and pass this through to KDialog (which defaults to accept), but that something is getting lost in the transition or that the revert is requested, but fails for some reason. 

This works in the krandrtray systray application, but it's code is completely different.