Corba was about the interoperability of platforms, but UML is about the interoperability of applications.
Interoperability being expressed at the abstract level is the key theme.
Whatever it takes to achieve end to end interoperability standards, we'll use, whether its our own or third parties. This is about being able to integrate the next big thing because we don't know what that will be. The reason for this is that, although enterprises originally introduced middleware to enable their heterogeneous systems to interoperate, no clear winner has emerged in this space and so many companies now have a raft of offerings in place. This enables application designers to construct, view, develop and manipulate models in a standard way so they can be evaluated before coding begins - when it is easier and cheaper to make changes. Although there will be several MDA core models, the first one to appear will be the Enterprise Computing model, which will handle the design of component and transaction-based applications.