We use domain-specific tasks to integrate with external services, and you can for example invoke a WSDL from inside the handler. Check out the documentation on domain-specific services for more details.
At this point, we don't (yet) support for example automatic generation of a new service task based on a WSDL interface. I don't think it would be too difficult to do though, and you can probably create one handler implementation that dynamically generates the WS invocation. And we would be interested in such extension! I can probably assist you, just find me on #jbpm on irc.codehaus.org
We're working on improving our WS integration, there's integration coming up with the SwitchYard project for example, so you can easily invoke web services that are deployed on the next generation ESB.