In Sep 2008, EMC, IBM and Microsoft had announced a jointly developed Content Management Interoperability Services (CMIS) specification. As a next step, an article authored by Trent Swanson, Bhushan Nene and Scot Hillier has been published in MSDN with details on integrating external repositories with SharePoint 2007. Here it is:
Integrating External Document Repositories with SharePoint Server 2007
Summary: Learn to design, implement, and deploy a complete solution for integrating an external document repository with Microsoft Office SharePoint Server 2007. External repositories can participate in workflows and use task lists and extensible metadata
You can also download the code samples: Integrating External Content Repository with SharePoint Server 2007
Want to know more on CMIS?
Read Press release: EMC, IBM and Microsoft Jointly Create First Web Services Interface Specification for Greater Interoperability of Enterprise Content Management Systems
"... EMC Corp., IBM Corp. and Microsoft Corp. today announced a jointly developed specification that uses Web Services and Web 2.0 interfaces to enable applications to interoperate with multiple Enterprise Content Management (ECM) repositories by different vendors. The companies intend to submit the Content Management Interoperability Services (CMIS) specification to the Organization for the Advancement of Structured Information Standards (OASIS) for advancement through its rigorous standards development process...."
Download a preview copy of the CMIS technical specification draft