Some folks believe that individual backbones will have their own services that may not interoperate. This runs contary to the central spirit of the Internet. There is a specification for extending BGP to carry protocol information. BGMP is another potential solution, but it is still early in its development cycle.
All connections are transit connections.
See a web page for some of the protocol interactions. They are not running yet,but real be happening soon. Run sparse mode backbones and dense mode interconnnects. There needs to be a concept of a border router to permit the sparse mode RP to exist correctly. To fix this, the testing beds will work to resolve this.