lighttpd.conf contains:

$HTTP["url"] =~ "^/forums" {
  fastcgi.server =  (".fcgi" => ( "beast" =>
    ( "min-procs" => 1,
      "max-procs" => 5,
      "socket" => "/tmp/pids/beast.socket",
      "bin-path" => "/var/www/frokdev/public/dispatch.fcgi"
    )
  ))
}

lighttpd breaks with this error message:

2007-05-04 22:59:30: (mod_fastcgi.c.1032) the fastcgi-backend /var/www/frokdev/public/dispatch.fcgi failed to start: 
2007-05-04 22:59:30: (mod_fastcgi.c.1036) child exited with status 1 /var/www/frokdev/public/dispatch.fcgi 
2007-05-04 22:59:30: (mod_fastcgi.c.1039) if you try do run PHP as FastCGI backend make sure you use the FastCGI enabled version.
You can find out if it is the right one by executing 'php -v' and it should display '(cgi-fcgi)' in the output, NOT (cgi) NOR (cli)
For more information check http://www.lighttpd.net/documentation/fastcgi.html#preparing-php-as-a-fastcgi-program 
2007-05-04 22:59:30: (mod_fastcgi.c.1044) If this is PHP on Gentoo add fastcgi to the USE flags 
2007-05-04 22:59:30: (mod_fastcgi.c.1340) [ERROR]: spawning fcgi failed. 
2007-05-04 22:59:30: (server.c.848) Configuration of plugins failed. Going down.