The following enhancement request has been logged regarding this issue:
NIM059964 Needs property or event to provide instantaneous placement information for dockable window when it is docked.
- IDockableWindow::State can tell whether the dockable window is docked or not.
- IDockableWindowInitialPlacement::DockPosition can only tell the initial placement of dockable windows
If possible please contact ESRI support and have your name added to the request.