This is a clean install of Ubuntu 12.10. I have tried to check Canonical Partners on the "Other" tab of the GUI for displaying/changing Software Sources in Software Center and in Synaptic. Both let me click in the box, request my password and nothing changes.
I have done this same thing in many previous releases and don't see any reason for it to fail now. I checked with someone else who may have just done an upgrade from 12.04 and his is checked but maybe it was before.
I could probably change it in the configuration file directly but then would not learn why the usual end user method is not working.