CMIS (Content Management Interoperability Specification) has been ratified as a standard by OASIS. What is it and what does it mean to buyers and users of ECM and Document Management technology? Well put simply, CMIS is the most important new standard in the ECM world in decades; it is critical for any enterprise scale RFP.
In essence CMIS allows different document management systems to interoperate at the repository level. So for example if you have a legacy system or two, or have plans to sunset a previous document management investment, you may be able to utilize CMIS to help with the migration of documents or to simply allow the old system to remain running, with documents at least accessible via the new system.
CMIS has the backing of most of the leading vendors in the space, and most have CMIS adapters available or in development. Any new system you buy should be CMIS compliant -- or on a sure path to compliance. Remember that your new system will itself become a legacy system one day, and it may well be beneficial to link it to other repositories at a future date.
There are much more complex, rich and impressive ways to integrate ECM systems together, and at times there is a need to leverage those methods;CMIS itself is no more than a base line.
For example CMIS is not a process interoperability standard, so don't assume it will give you business-logic integration; also, the first draft of the spec necessarily focuses on basic repository operations and not more advanced services, which may or may not come in subsequent version. So CMIS is not a complete panacea.
Nevertheless, it's a standard of more importance to enterprises than it is to vendors, who rather like the idea of locking you into their way of doing things. Hence my enthusiastic encouragement to you to include CMIS support as a requirement on any future ECM or Document Management RFP.