The good news about service-oriented architectures (SOA) is the flexibility they provide for ad hoc integration within and across enterprises. It's also the bad news when it comes to testing, which relies on predictability and repeatability.
The key is to first isolate the architectural layers so that functionality can be tested -- and issues exposed -- at their source as early as possible, then to gradually integrate the components until end-to-end business processes can be verified.
Equally important is to engage subject matter experts throughout the development life cycle to ensure compliance not only with technical specifications but also with business requirements. Waiting for business process acceptance testing to be performed when all the layers are complete is to invite disaster and delay: a service that returns the wrong result in the right way is perhaps even more dangerous than one which fails to execute altogether because it has a higher risk of being undetected, and uncovering the root source of a problem takes longer the more layers are involved.
