As of Release: 0.9.15 [18 Jan, 2013]
there's a new feature to show/hide controls in viewer window. Problem is I can't find where it is.
E-book viewer: Add an option to hide the toolbars in the viewer window (Preferences->Miscellaneous->Show controls in the viewr preferences). You can unhide them by right clicking in the viewer window.
Under Preferences>Miscellaneous I only see irrelevant options.
I'm on Win7 with Calibre portable just updated to this version.