Running "apt-get update" with the version of apt in unstable will probably display the following (non-fatal) errors if you're using unofficial apt sources:

W: GPG error: http://apt.example.com unstable Release: The following signatures
couldn't be verified because the public key is not available: 
NO_PUBKEY A35A4E6EF00175CA
W: You may want to run apt-get update to correct these problems

In order to fix this you need to import the key into gpg, then export the key into apt, using "apt-key".

$ gpg --keyserver wwwkeys.eu.pgp.net --recv-keys A35A4E6EF00175CA
gpg: requesting key F00175CA from hkp server wwwkeys.eu.pgp.net
gpg: key F00175CA: public key "Sebastien NOEL " imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1
$ gpg --armor --export A35A4E6EF00175CA | sudo apt-key add -
gpg: no ultimately trusted keys found
OK