I have a couple of customers using commercially available plugins from: http://netapp-monitoring.info/en/nagios-plugins.html
Ingo, the developer behind these plugin has been really helpful with evals etc.
I was in a customer project, playing around with forwarding DFM alerts to Nagios using passive checks using a perl kit I got from Keith Tenzer. It didn't take long until I realized this would become overly complicated and need filtering and or logic implemented in either side - DFM or Nagios. For example, let's say a disk fails. An event is triggered and turns the disk icon in Nagios red. Another disk fails. Another event is sent. Icon is still red. Now you replace one of the two broken disks. Event is sent. But how do you know if you can turn the icon green again? You need to keep track of multiple events and it's a mess.
That's when we discovered the plugins above and never looked back.
I also have customers using the stock standard plugins found on the 'net.
Hope this helps,