OP postado:
Depois de fazer a pergunta, eu tentei o procedimento a seguir e funcionou, então talvez possa ser útil para os outros: Eu tinha no arquivo source.list este erro:
52 ## respective vendors as a service to Ubuntu users.
53 # deb http://archive.canonical.com/ubuntu precise partner
54 # deb-src http://archive.canonical.com/ubuntu precise partner
e
59 deb http://archive.canonical.com/precise partner
60 deb-src http://archive.canonical.com/precise partner
Eu os alterei da seguinte forma:
52 ## respective vendors as a service to Ubuntu users.
53 deb http://archive.canonical.com/ubuntu precise partner
54 deb-src http://archive.canonical.com/ubuntu precise partner
...
59 # deb http://archive.canonical.com/precise partner
60 # deb-src http://archive.canonical.com/precise partner
E agora tudo funciona!