In prepping for the 12.04.3 release I ran across a problem where the keyring package could be de-installed since it was being classified as "not needed" by APT.
If deleted, it would result in the source files for the Centyrch software repositories also being deleted. During the re-organization I didn't update the settings package properly, so the source files, while not deleted, will get reset.
While the reset is unavoidable, these two packages will not reset the files when future updates are released.
The upshot is that you'll need to go into Software Sources and re-activate the repositories you were previously using.