I have designed UML diagrams in different tools (StarUML, BoUML, Papyrus, Omondo, Rational Rose, etc.) depending on the project and the date (rose was first, after Omondo, t
Besides Jordi's reference to the Diagram Definition standardization activity, please note that there is another action ongoing from OMG: a request for comments has been issued in June 2011 for a CANONICAL XMI format, which is basically a subset and a set of additional rules for writing XMI documents that should limit the possibility of creating different XMI dialects, or at least should allow tools to understand a common dialect (the canonical one).