Index: /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxGlobal.cpp
===================================================================
--- /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxGlobal.cpp	(revision 26188)
+++ /trunk/src/VBox/Frontends/VirtualBox/src/globals/VBoxGlobal.cpp	(revision 26189)
@@ -4760,4 +4760,6 @@
     /* process command line */
 
+    bool bForceSeamless = false;
+
     vm_render_mode_str = RTStrDup (virtualBox()
             .GetExtraData (VBoxDefs::GUI_RenderMode).toAscii().constData());
@@ -4805,4 +4807,8 @@
             }
         }
+        else if (!::strcmp(arg, "-seamless") || !::strcmp(arg, "--seamless"))
+        {
+            bForceSeamless = true;
+        }
 #ifdef VBOX_GUI_WITH_SYSTRAY
         else if (!::strcmp (arg, "-systray") || !::strcmp (arg, "--systray"))
@@ -4862,4 +4868,9 @@
         /** @todo add an else { msgbox(syntax error); exit(1); } here, pretty please... */
         i++;
+    }
+
+    if (bForceSeamless && !vmUuid.isEmpty())
+    {
+        mVBox.GetMachine(vmUuid).SetExtraData(VBoxDefs::GUI_Seamless, "on");
     }