Rok,
Empirical evidence seems to show that ssh will honor a request to do X11 forwarding only if it finds $DISPLAY is set in the calling process (iTerm in your case, or the shell when you use the command line).
To set DISPLAY in iTerm you need to use PropertyListEditor.app to add an entry in .MacOSX/environment.plist, then log out and back in. See
http://developer.apple.com/qa/qa2001/qa1067.html
The value I gave DISPLAY was ":0.0" which works for me.
PatB2