Editei o sources.list com o gedit, e o substitui pela sugestão do Edinaldo P. Silva:
# deb cdrom:[Ubuntu 11.04 _Natty Narwhal_ - Release i386 (20110427.1)]/ natty main restricted
"# Ubuntu 11.04 Oneiric.
deb
http://mirror.rit.edu/ubuntu/ oneiric main restricted universe multiverse
#deb
http://br.archive.ubuntu.com/ubuntu/ oneiric main restricted universe multiverse
# Ubuntu 11.04 Oneiric (Security Updates).
deb
http://mirror.rit.edu/ubuntu/ oneiric-security main restricted universe multiverse
#deb
http://br.archive.ubuntu.com/ubuntu/ oneiric-security main restricted universe multiverse
# Ubuntu 11.04 Oneiric (Updates).
deb
http://mirror.rit.edu/ubuntu/ oneiric-updates main restricted universe multiverse
#deb
http://br.archive.ubuntu.com/ubuntu/ oneiric-updates main restricted universe multiverse
# Ubuntu 11.04 Oneiric (Backports).
deb
http://mirror.rit.edu/ubuntu/ oneiric-backports main restricted universe multiverse
#deb
http://br.archive.ubuntu.com/ubuntu/ oneiric-backports main restricted universe multivers
# Ubuntu 11.04 Oneiric (Partner).
deb
http://archive.canonical.com/ubuntu oneiric partner
# Ubuntu 11.04 Oneiric (Extras).
deb
http://extras.ubuntu.com/ubuntu oneiric main
# Ubuntu 11.04 Oneiric (Medibuntu).
# ChaveGPG: sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo apt-get update
deb
http://packages.medibuntu.org/ oneiric free non-free"
Porém sou informado que não tenho permissão para salvá-lo:
Não foi possível salvar o arquivo /etc/apt/sources.list. Você não tem as permissões necessárias para salvar este arquivo. Por favor, verifique se você digitou a localização corretamente e tente de novo.
Aqui, o resultado do comando sudo ls /etc/apt/sources.list.d/:
akirad.list
chromium-daily-ppa-oneiric.list
chromium-daily-ppa-oneiric.list.save
chromiun-daily-ppa-oneiric.list
chromiun-daily-ppa-oneiric.list.save
cinelerra-ppa-ppa-oneiric.list
cinelerra-ppa-ppa-oneiric.list.save
conscioususer-polly-daily-oneiric.list
conscioususer-polly-daily-oneiric.list.save
medibuntu.list
medibuntu.list.save
oneiric-partner.list
oneiric-partner.list.save
Como disse, sou usuário básico!
O quê fazer agora?