Comment 4 for bug 67152

Andreas reports that he makes the changes in the "interface properties" dialog box, then clicks OK but does not see the changes made in the /etc/network/interfaces config file.

However after that Interface Properties dialog box closes, focus is returned to network-admin dialog box (titled "Network settings") at that point, I too see that the config file has not yet been written to. It is not until the OK button in network-admin also is pressed that network-admin writes the changes to the config file.

If instead you click cancel in network-admin, any changes made in Interface Properties are discarded and never written to the config file.

Additionally, I am experiencing a similar problem in connecting where after making the change (even if network-admin writes the changes to the config file), I still need to manually re-issue the iwconfig to get a connection, or else I can simply reboot:
https://bugs.launchpad.net/ubuntu/+source/netbase/+bug/80499