One day there will be a single BIM. This will be created when the client is briefing the design team. All project members will work on this single shared BIM through the lifecycle of the building. For more information on this concept see the website http://bimserver.org externallink.
However, this technology is in its infancy. In the near future, it is more likely that there will be a number of BIMs and that data from each will interoperate. Each designer will have their own CAD and specification models. The data from each will come together at key milestones to allow costing, quantity take off, performance analysis and clash detection. Finally this data will all be exported to a non-proprietary data format such as BS 1192-4 for handover to the building owner.
The current focus should not be on taking the innovators or the early adopters to a "holy grail" of a single BIM server, but to get the majority of the industry to adopt the key advantages of BIM.