There are two possibilities to deal with this issue: Install the packages debian-keyring and debian-archive-keyring to avoid the warning or simply ignore the warning and install untrusted packages. Of course you will have to override the warning when you install them. Here is what I did:
Misi:~# aptitude install debian-keyring
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done
The following NEW packages will be installed:
debian-keyring
0 packages upgraded, 1 newly installed, 0 to remove and 267 not upgraded.
Need to get 31,1MB of archives. After unpacking 35,8MB will be used.
WARNING: untrusted versions of the following packages will be installed!
Untrusted packages could compromise your system's security.
You should only proceed with the installation if you are certain that
this is what you want to do.
debian-keyring
Do you want to ignore this warning and proceed anyway?
To continue, enter "Yes"; to abort, enter "No": yes
Writing extended state information... Done
Get:1 http://ftp.us.debian.org squeeze/main debian-keyring 2010.12.29 [31,1MB]
Fetched 31,1MB in 31s (977kB/s)
Wähle vormals abgewähltes Paket debian-keyring.
(Lese Datenbank ...
dpkg: Warnung: Dateilisten-Datei des Paketes »ramlog« fehlt, nehme an, dass das Paket derzeit keine Dateien installiert hat.
dpkg: Warnung: Dateilisten-Datei des Paketes »libparted0« fehlt, nehme an, dass das Paket derzeit keine Dateien installiert hat.
(Lese Datenbank ... 24014 Dateien und Verzeichnisse sind derzeit installiert.)
Entpacke debian-keyring (aus .../debian-keyring_2010.12.29_all.deb) ...
Richte debian-keyring ein (2010.12.29) ...
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Writing extended state information... Done
Reading task descriptions... Done
Misi:/DataVolume/shares/Daten/.AppleDB# aptitude install debian-archive-keyring
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done
The following packages will be upgraded:
debian-archive-keyring
1 packages upgraded, 0 newly installed, 0 to remove and 266 not upgraded.
Need to get 19,9kB of archives. After unpacking 4096B will be used.
Do you want to continue? [Y/n/?] y
WARNING: untrusted versions of the following packages will be installed!
Untrusted packages could compromise your system's security.
You should only proceed with the installation if you are certain that
this is what you want to do.
debian-archive-keyring
Do you want to ignore this warning and proceed anyway?
To continue, enter "Yes"; to abort, enter "No": yes
Writing extended state information... Done
Get:1 http://ftp.us.debian.org squeeze/main debian-archive-keyring 2010.08.28 [19,9kB]
Fetched 19,9kB in 0s (20,8kB/s)
(Lese Datenbank ...
dpkg: Warnung: Dateilisten-Datei des Paketes »ramlog« fehlt, nehme an, dass das Paket derzeit keine Dateien installiert hat.
dpkg: Warnung: Dateilisten-Datei des Paketes »libparted0« fehlt, nehme an, dass das Paket derzeit keine Dateien installiert hat.
(Lese Datenbank ... 24025 Dateien und Verzeichnisse sind derzeit installiert.)
Vorbereiten zum Ersetzen von debian-archive-keyring 2009.01.31 (durch .../debian-archive-keyring_2010.08.28_all.deb) ...
Entpacke Ersatz für debian-archive-keyring ...
Richte debian-archive-keyring ein (2010.08.28) ...
gpg: key F42584E6: "Lenny Stable Release Key <debian-release@lists.debian.org>" not changed
gpg: key 55BE302B: "Debian Archive Automatic Signing Key (5.0/lenny) <ftpmaster@debian.org>" not changed
gpg: key 6D849617: "Debian-Volatile Archive Automatic Signing Key (5.0/lenny)" not changed
gpg: key B98321F9: public key "Squeeze Stable Release Key <debian-release@lists.debian.org>" imported
gpg: key 473041FA: public key "Debian Archive Automatic Signing Key (6.0/squeeze) <ftpmaster@debian.org>" imported
gpg: Total number processed: 5
gpg: imported: 2 (RSA: 2)
gpg: unchanged: 3
gpg: no ultimately trusted keys found
Reading package lists... Done
Building dependency tree
Reading state information... Done
Reading extended state information
Initializing package states... Done
Reading task descriptions... Done
Current status: 266 updates [-1].
Misi:~# aptitude update
Get:1 http://ftp.us.debian.org squeeze Release.gpg [1671B]
Hit http://ftp.us.debian.org squeeze/main Translation-de
Hit http://ftp.us.debian.org squeeze Release
Ign http://ftp.us.debian.org squeeze/main Packages/DiffIndex
Hit http://ftp.us.debian.org squeeze/main Packages
Fetched 1671B in 1s (1578B/s)
Reading package lists... Done
You can also ignore the warning and go on installing untrusted packages as I did. Anyway it hasn't affected system stability yet, but your post made me to install them, so this nasty message has disappeared now. Maybe I will write a little tutorial to summarize what I have learned so far.