Mano eu estava com o mesmo problema e seguindo a dica do platão eu resolvi, tenta ai...
http://ubuntuforum-br.org/index.php/topic,95441.msg524718.html#msg524718
Olá, Kadu.
Eu segui a sugestão do Tota, mas não surtiu efeito e apareceu uma outra mensagem gráfica.
"update-manager: Falha ao baixar informações do repositório."
Então segui a sugestão do Platão, como você indicou, mas gerou um relatório ainda cheio de erros, conforma abaixo.
Obrigado por enquanto.
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E: Alguns arquivos index falharam ao ser baixados. Eles foram ignorados, ou cópias antigas são usadas ao invés.