CIOs can't use a single method: It depends on what you are trying to communicate. What are you trying to communicate and who is your audience and how do you personally communicate best? All of the methods mentioned above should be used as well as a regular formal process that shows they're working together rather than independently.
Using an informal discussion is a good way to start and continue a dialogue: This approach will allow for authenticity and credibility to build relationships of trust. That is the 'currency' of executive management... Informal Discussions with executives guarantee to address individual organizations' IT requirements and provide the great insight of their views and feedback, but then you also need other methods that allow your fellow executives to answer their own questions or refresh their memory such as utilizing a portal with documents, presentations, etc.
A combination of IT briefings and informal conversation helps bring interpersonal communication more effective: A CIO needs to communicate with business executives for different reasons. There is a need for formal communications paths for gaining approval, but before that happens, there must be a level of rapport or trust between executives that can only be built through informal communications. Then there must also be regular, formal status communications, but the informal paths are critical in making sure that the formal communications aren't misinterpreted. For example, the information portals are good tools for providing information on an ad hoc basis as business executives need it, but it shouldn't rely on as the primary or only source of communications.
It is not just Boards - the research shows that the C-suite is hungry for the CIO to step up with their knowledge of the business. Information and insight are just two of the reasons for being at the table. Meaningful exchanges and engagement will deliver competitive advantage