Buenas, he querido actualizar el sistema, uso debian squeeze y obtuve un error que nose que es, les muestro el error a continuacion:

Sudo aptitude update:

Des:1 http://www.debian-multimedia.org testing Release.gpg [198 B]
Ign http://www.debian-multimedia.org/ testing/main Translation-en
Ign http://www.debian-multimedia.org/ testing/main Translation-es
Ign http://www.debian-multimedia.org/ testing/non-free Translation-en
Obj http://ftp.debian.org testing Release.gpg
Ign http://ftp.debian.org/debian/ testing/contrib Translation-en
Ign http://ftp.debian.org/debian/ testing/contrib Translation-es
Obj http://security.debian.org testing/updates Release.gpg
Ign http://security.debian.org/ testing/updates/contrib Translation-en
Ign http://security.debian.org/ testing/updates/contrib Translation-es
Ign http://www.debian-multimedia.org/ testing/non-free Translation-es
Des:2 http://www.debian-multimedia.org testing Release [29,4 kB]
Ign http://ftp.debian.org/debian/ testing/main Translation-en
Obj http://ftp.debian.org/debian/ testing/main Translation-es
Ign http://ftp.debian.org/debian/ testing/non-free Translation-en
Ign http://ftp.debian.org/debian/ testing/non-free Translation-es
Obj http://ftp.debian.org testing Release
Ign http://www.debian-multimedia.org testing Release
Ign http://security.debian.org/ testing/updates/main Translation-en
Ign http://security.debian.org/ testing/updates/main Translation-es
Ign http://security.debian.org/ testing/updates/non-free Translation-en
Ign http://security.debian.org/ testing/updates/non-free Translation-es
Obj http://security.debian.org testing/updates Release
Obj http://www.debian-multimedia.org testing/main amd64 Packages/DiffIndex
Obj http://ftp.debian.org testing/main Sources/DiffIndex
Obj http://security.debian.org testing/updates/main Sources
Obj http://www.debian-multimedia.org testing/non-free amd64 Packages/DiffIndex
Obj http://ftp.debian.org testing/contrib Sources/DiffIndex
Obj http://ftp.debian.org testing/non-free Sources/DiffIndex
Obj http://ftp.debian.org testing/main amd64 Packages/DiffIndex
Obj http://security.debian.org testing/updates/contrib Sources
Obj http://security.debian.org testing/updates/non-free Sources
Obj http://security.debian.org testing/updates/main amd64 Packages
Obj http://security.debian.org testing/updates/contrib amd64 Packages
Obj http://security.debian.org testing/updates/non-free amd64 Packages
Obj http://ftp.debian.org testing/contrib amd64 Packages/DiffIndex
Obj http://ftp.debian.org testing/non-free amd64 Packages/DiffIndex
Descargados 199 B en 2s (73 B/s).
W: GPG error: http://www.debian-multimedia.org testing Release: Las firmas siguientes no se pudieron verificar porque su llave pública no está disponible: NO_PUBKEY 07DC563D1F41B907

Si les sirve mis repositorios

deb http://security.debian.org/ testing/updates main contrib non-free
deb-src http://security.debian.org/ testing/updates main contrib non-free
deb http://www.debian-multimedia.org testing main non-free
deb http://ftp.debian.org/debian testing main contrib non-free
deb-src http://ftp.debian.org/debian testing main contrib non-free

Gracias