In Update1 a new data sharing feature has been added. You can use the feature to share simple group and reference data between companies in an AX deployment.
You can read more about the feature in the Microsoft Dynamics AX Wiki.
From a technical perspective, it is interesting to see how the setup generates triggers on the database to register changes and to replicate data. These also indicates the caveat; data is replicated on SQL level and AX business logic is not executed.