The action handler mechanism is indeed the way to call out to external applications. You can add actions in nodes but also when certain events happen like going over a transition, entering and leaving a node.
The actions in nodes mostly combine process control flow and other 'business' behaviour. Actions on transitions or other events in general do not modify the process control flow.
We are still hoping for the community to develop and/or contribute some generic actionhandlers like calling webservices, sending email, retrieving info from databases or ERP systems. Until now not much has happened on this front. Starting from the latest beta2 release we do have a special action handler that is able to evaluate jsf style expressions which is really nice.