Content Management Interoperability System.
Had a quick look at this for a client last week. Managed to get it working with Documentum from .NET clients – a bit slow but it does work.
I used the DotNET client from openCMIS – Apache Chemistry. http://incubator.apache.org/projects/chemistry.html
DotCMIS is the best thing I found – although I really struggled to get web services working with Documemtum – so I used the AtomPub binding and the session factory.- that seemed to work reasonably well.