An important topic when designing service oriented systems is how to enable different services to share semantics to be able to be composed into working solutions. Jack van Hoof has written a good article about this: How to mediate semantics in an EDA. A few weeks later, Nick Malik posted another good read about this topic: Canonical Model, Canonical Schema, and Event Driven SOA. Read Jack's post first.
They both talk about using a canonical data model (CDM) as the Esperanto / Babel fish to map between the format and semantics of the disparate systems taking part in a SOA solution. Note that CDM is not about having a common data model (EAI CDM) or a shared database across all systems in an enterprise, don't get fooled by the "data" in the term 'canonical data model'. CDM is about not making everybody have to speak English, but rather having CDM translators for each native system.
Btw, Gregor Hohpe sometimes use the term 'canonical domain model' on his blog, while using the term 'canonical data model' in the book "Enterprise Integration Patterns" [Hohpe/Woolf CDM pattern (355)]. I think it is better to talk about the business domain rather than about "data", as this help focusing on the business processes rather than databases and other technology. You'd be surprised how many biz people concern themselves with how the data model looks - maybe a leftover from the client-server days, to show that they know what an ER-diagram is? Focus on designing a business process information model (BPIM) for each business process domain.
Trying to enforce a One True Schema across your services (everyone has to speak English) is not a viable path, and it is also a recipe for future maintenance hell. Making every service contract depend on the One True Schema will make it impossible for the services to evolve separately, they will no longer be autonomous. A simple change to e.g. the order entity will cause a ripple effect through all referring services. This is where the business process information model comes into play, it allows you to version and evolve the services independently of each other.
The Canonical "Data" Model concept is also sometimes referred to as a Common Information Model (CIM). Both the business process information model (BPIM) and the data focused CDM/CIM models has the same goal: mediation og semantics. However, they are not the same as the two other models are both variations of the common data model approach. The business process information model is about semantic business process integration, not just only semantic data integration.