Correction: My metacity binds <Alt>tab to switch-window. This seems to be the default; see also
http://www.gungeralv.org/notes/archives/000323.php .
In contrast, <Control>tab is not interpreted by the window manager. It gets passed to any
X client, e.g., firefox (next tab in tabbed browsing) or vncviewer. I just did the test with
the latter. BTW,
xev is a great tool for figuring such things out, because it lets
you see what X11 events are sent to client windows.
Good luck!
/Quigi