A couple of posts on this subject recently, one of them by my good friend John Reynolds (Process Manager Interoperability).? Keith Swenson made a reference to the Wf-XML spec as being relatively little-known and he wasn't kidding - neither John nor I had heard of it previously and no doubt part of that is because the name says nothing about what it can accomplish for you.? The letters "Wf-XML" just don't say anything about what this spec will do for you (allow you to start, monitor, and react to the completion of a process run in another system/environment).
As John points out, it seems higher probability that consumers of BPM technologies will benefit more from technologies that help with interoperability between those BPM tools, rather than from the more complicated task of actually migrating those implementations across BPM tools.? Of course, the SOA camp ca well claim that they solve this problem - but it isn't the service-oriented nature of the interface that is the problem - its having a purpose-built standard for the design pattern of two independent process platforms communicating about the state of a process instance (or subprocess instance).