Keith recently listed 21 questions in his blog that he recommends a prospective buyer should ask BPM vendors when comparing products.? Not all the questions are about interoperability exclusively, but the first few are definitely focused on interoperability.? However, you have to weight questions based on how important they are to you.? You can get a highly inter-operable product that doesn't functionally *do* what you need it to do.? You can get a product that functionally hits all your needs but doesn't support interchange formats... and you can find data points inbetween.
Unfortunately we're still very early in the world of interchanging business process models.? I would be hesitant to place interchange high on my list for differentiating my vendors.? I would place more emphasis on functional requirements, total cost of ownership, and adherence to my IT approach.? After those needs are met, then I'd start sorting by other things (interchange of BPM models being one).? One of the things I've noticed is that the general assumption seems to be that if I export a business process from one tool to XPDL and import it to another, it will just run in the new tool.? That isn't necessarily the case.? Different BPM tools provide a different level of support for self-contained implementation specifications (implementation of those activities or services), and generally speaking those implementation details won't translate, because they aren't explicitly part of any of the BPM specs.
Having said that, Keith has a few questions to ask that have an impact on interoperability or interchange, but are also key differentiators of product capability, including questions about nested processes and how deep the nesting can go, data tracking, event posting, supporting wf-xml, exposing processes as webservices, etc.? Check out his list for more in-depth discussion of each one!