EDIT: CruiseControl.NET version 1.3 now ships with an integrated FinalBuilder Task. No more need for a plugin.
A few people have been enjoying this for a while, but you can now download an installer for the FinalBuilder CruiseControl.NET plugin. The plugin makes FinalBuilder available as a CruiseControl.NET build engine for continuous integration. The plugin is free, and works with both FinalBuilder 3 and 4 (although FB 3 users will need the Professional Edition.)
The current version of the plugin is for CruiseControl.NET 1.0, and we will make sure it keeps up to date with the current stable release.