The checkboxes that appear to be changeable under the Edit / Software Sources dialog box cannot be changed. I click on them and they just turn gray and stay at their current setting.

Update:

When I run software-center from a terminal window and try to change one of the checkbox settings, I get:

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/softwareproperties/gtk/SoftwarePropertiesGtk.py",
        line 649, in on_isv_source_toggled
    self.backend.ToggleSourceUse(str(source_entry))
  File "/usr/lib/python2.7/dist-packages/dbus/proxies.py", line 143, in __call__
    **keywords)
  File "/usr/lib/python2.7/dist-packages/dbus/connection.py", line 630, in call_blocking
    message, timeout)
dbus.exceptions.DBusException:
  com.ubuntu.SoftwareProperties.PermissionDeniedByPolicy:
  com.ubuntu.softwareproperties.applychanges

These things happen instead of it properly prompting me for a password (for root privs).