If you're using Ubuntu 12 LTS "Precise Pangolin" with the Nagios 3.4.1 package from the Ubunutu repository and you're having trouble with passive checks and the freshness_threshold directive you may want to consider upgrading Nagios. Version 3.4.1 suffers from a bug that prevents passive checks and the freshness_threshold directive from properly working as documented if the freshness_threshold is more than a couple hours. This forum posting tipped me off to the problem and an upgrade to 3.5.1 from the latest tarball resolved the problem.
In my case the goal was to implement passive checks with freshness_thresholds longer than 24 hours. Testing the configuration with the freshness_threshold set to less than 10 minutes worked perfectly but once the threshold was increased active checks never triggered after the threshold was passed leaving the service or host in a continue "OK" state.
I Hope this saves you a from a lot of searching and head scratching. If you've found this post and still haven't gotten your configuration working I recommend you go to the Nagios Support Forums and search. The forums have a lot of relevant information that don't always come up when searching directory from Google.