Folks:
You can also look at:
http://archicad-talk.graphisoft.com/viewtopic.php?p=23162&highlight=hide#23162
From my dim memory: If you started with a recent default keyboard shortcut scheme (ArchiCAD 8 or 9), it was not possible to assign Command H to anything (the application will hide when imputing the key combination in the Work Environment, of course). For users wanting Command H for Stretch (ArchiCAD 5 or 6), they needed to go to the Work Environment and start with an older scheme that used Command H by default. I think if you start with an older scheme, Command H is gone for Hiding ArchiCAD, and will not come back even if you remove Command H from a keyboard shortcut. I just tried exporting my own Profile, but Command H remained for hiding ArchiCAD.
Hope this helps.
David