We therefore decided that we should also monitor the validity of signatures online. To achieve this, we created a small tool that plugs in to Nagios and that can check the validity time of the signatures for either a single resource record or for a whole zone using an AXFR-style transfer.
You can download this tool using the link below; the source distribution includes a README with instructions on building and using the tool. The tool is released under a BSD-style license (included).
Download the tool here: sigvalcheck-0.1.tar.gz
UPDATE: The trunk of OpenDNSSEC also includes a very useful monitoring tool that integrates in Nagios; it is written in Ruby and available through the OpenDNSSEC subversion repository.