I know this thread is quite old but I think it's worth mentioning for people finding this.
We had a good experience using Arduino to run two different systems on a digital printing machine - there's a fully write up at Arduino Isn't Just For Hackers
As you can see from the linked discussions there are a lot of disagreements around whether Arduino is suitable in general for industrial use. However in my view it really can't be generalised - there's a whole spectrum of applications with different requirements. In this particular case it was a good fit (and still is as of August 2014).
I'm not involved any more but I'm sure my dad (who runs JF Machines) would be happy to answer any questions about their more recent experience: http://www.jfmachines.co.uk.