Eu estou com o mesmo problema. Esse foi o resultado do comando sudo apt-get update:
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http://extras.ubuntu.com oneiric Release.gpg [71 B]
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http://archive.ubuntu.com oneiric Release.gpg [198 B]
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http://archive.ubuntu.com oneiric-updates Release.gpg [198 B]
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http://archive.ubuntu.com oneiric-backports Release.gpg [198 B]
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http://archive.ubuntu.com oneiric-security Release.gpg [198 B]
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http://archive.ubuntu.com oneiric/universe i386 Packages [4.468 kB]
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http://archive.ubuntu.com oneiric/main Translation-pt_BR [293 kB]
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http://archive.ubuntu.com oneiric/multiverse Translation-pt_BR [22,9 kB]
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http://archive.ubuntu.com oneiric/restricted Translation-pt_BR [2.272 B]
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http://archive.ubuntu.com oneiric/universe Translation-pt_BR [731 kB]
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Baixados 5.519 kB em 5min 35s (16,5 kB/s)
Lendo listas de pacotes... Pronto
W: Um erro ocorreu durante a verificação da assinatura. O repositório não foi atualizado eo índice anterior de arquivo será usado. Erro GPG:
http://extras.ubuntu.com oneiric Release:As seguintes assinaturas eram inválidas: BADSIG 16126D3A3E5C1192 Ubuntu Extras Archive Automatic Signing Key <ftpmaster@ubuntu.com>
W: Erro GPG:
http://archive.ubuntu.com oneiric Release: As seguintes assinaturas eram inválidas: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
W: Erro GPG:
http://archive.ubuntu.com oneiric-updates Release: As seguintes assinaturas eram inválidas: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
W: Erro GPG:
http://archive.ubuntu.com oneiric-backports Release: As seguintes assinaturas eram inválidas: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
W: Erro GPG:
http://archive.ubuntu.com oneiric-security Release: As seguintes assinaturas eram inválidas: BADSIG 40976EAF437D05B5 Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>
W: Falhou ao buscar
http://extras.ubuntu.com/ubuntu/dists/oneiric/Release W: Alguns arquivos index falharam ao ser baixados. Eles foram ignorados, ou cópias antigas são usadas ao invés.