When installing the radiant gem and running using apache/passenger (mod_rails) be sure to chmod -R 777 cache where cache is a directory in the root folder. Otherwise, you may get an error like this:

[GET]

Parameters: {"url"=>"/", "action"=>"show_page", "controller"=>"site"} Completed in 240ms (DB: 8) | 200 [http://www.workingwithdata.com/] /!\ FAILSAFE /!\ Mon Jan 2 06:13:04 +0000 2009 Status: 500 Internal Server Error Permission denied - /home/demo/etc/rails_apps/mydomain/cache/entity/buf-2894985-4904395

when apache tries to write to your site cache. Also, check the ownership of the cache and it's subdirectories, too.