If you get the following error message after installing xen tools and running aptitude update

W: GPG error: http://updates.vmd.citrix.com etch Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 841D6D8DFE3F8BB2
W: You may want to run apt-get update to correct these problems

Issue the following command:

wget -q http://updates.vmd.citrix.com/XenServer/5.5.0/GPG-KEY -O- | apt-key add -

This will download the current gpg key from the citrix update server and add the key to apt. You should see an ok message after thats done run

aptitude update The error message should be gone