I want to let you know about an issue with the Nagios3 packages in the Lenny-backports AMD64 repository. All of the Nagios3 packages are version 3.2.0-4~bpo50+1 except nagios3-common and nagios3-doc which are version 3.2.0-5~bpo50+1. This issue is causing version conflicts that disallow the installation of Nagios3. I hope this helps, and please let me know if/when it is resolved. Thanks, Jeremy