Tenable has made an RSS feed of vulnerability plugin updates available for the Passive Vulnerability Scanner. The feed is located at:

http://www.tenablesecurity.com/pvs.xml

Tenable customers who manage their PVSs with the Security Center automatically synchronize with the latest checks daily. However, if a new vulnerability check is available that is of interest, you can force an update under the "Force Nessus Plugin Update" link under the "Policies" tab.

Tenable also offers a feed for Nessus plugins located at:

http://www.nessus.org/rss-plugins.xml

This includes information about plugins from both the direct and registered feeds.