You can find more information about Tuscany/SCA from this article:
http://java.sys-con.com/node/458183
A few points:
1) Tuscany doesn't have to be exclusive against other stacks such as Camel or ServiceMix. We can plugin Camel as a mediation component into SCA. We can build a binding for ServiceMix/Mule. ESBs can also use SCA as the component model (similar as Mule flow or ServiceMix's spring integration to encapsulate business logic connected to the endpoints).
2) Tuscany specializes in componentizing the business logic and composing them in a fairly technology neutral fashion. With Tuscany, most of the developers won't touch the technology APIs. They are just writing business logic and the Tuscany runtime takes care of the rest.
3) SCA has the domain concept that provides location transparency and service management in the distributed environment.
4) Tuscany adapts to various protocols using the declarative binding concept. It doesn't require normalized messages as most ESBs does.
5) SCA has a policy model that allows the abstraction of QoSs.
In a nutshell, Tuscany SCA does the job to guide you decompose business problems, build reusable blocks, and compose them by wiring. The components are also connected to outside services or clients via the protocol bindings.
Some links FYI:
http://entarch.blogspot.com/2008/05/decision-makers-concern-about-esb.html
http://entarch.blogspot.com/2008/05/enterprise-service-bus-vs-service.html