I have a lot of papers printed out and a year later I don't know which I already have printed. I think it might be helpful to provide an "open-to-print" option additionally to the open file option. This option would just set a "Printed" attribute to the file. Of course it would only open the file, but thats up to the user. If any pdf viewer provides an interface to start the printer dialog directly, this could improve the feature, but it is not necessary.
Fixed and merged. Pull request #28
https://github.com/JabRef/jabref/pull/28