The nm-applet in NetworkManager-gnome-0.7.0-0.3.svn2983.fc8 always has the "Connect to Other Wireless Network" option greyed out, preventing the user from connecting to a hidden network.
Should be all hooked up now, can you test with svn3016 from koji?
Dialogs work for me, but there's no hidden networks for me to test with (that I know about..) so I can't confirm that it does the right thing. But this bug was soley about the GUI, so that's sufficient.