It will be extremely useful to have a DBus interface for display manager.  This
would allow gdm, gnome-screensaver, and fast-user-switch-applet to interoperate
well and to share more code.  James Cape has drafted a possible interface:

DisplayManager {
  Display[] ListDisplays           (void);
  Display   GetDisplayByName       (string name);
  Display   GetDisplayByConsole    (int vtnum);
  Display   GetActiveDisplay       (void);
  Display   GetCurrentDisplay      (void);
  User[]    GetValidUsers          (void);

  bool      Shutdown               (int how, int when);
  bool      ActivateDisplay        (Display display);

  void      "user-added"           (User);
  void      "user-removed"         (User);
  void      "display-added"        (Display);
  void      "display-removed"      (Display);
};

Display {
  string    GetName                (void);
  int       GetConsole             (void);
  bool      GetIsNested            (void);
  User      GetUser                (void);

  bool      StartLogin             (string username);

  void      "user-changed"         (User);
}

User {
  string    GetName                (void);
  uid_t     GetUid                 (void);
  string    GetDisplayName         (void);
  string    GetPhotoUri            (void);
  Display[] GetDisplays            (void);

  void      "name-changed"         (void);
  void      "uid-name-changed"     (void);
  void      "display-name-changed" (void);
  void      "photo-changed"        (void);
  void      "displays-changed"     (void);
}