I have the evaluation copy of the µVision IDE/Debugger V2.10. I have undocked the output window (while not debugging) and the damn thing refuses to dock again. While debugging I can dock and undock the output window fine.
Never mind I found the problem, pilot error. If you undock and output window and right click on the title bar it displays a small menu for move and hide. However if you right click within the window itself it displays another menu to enable docking.