Enterprise IT environments are a mess primarily because they've grown piecemeal by function and technology. Every business unit and indeed every project is "empowered" to make their own decisions. And so they rightly make the best decision for their purposes. Every vendor has some hook in their technology to bring you back to the trough.
Even if some Enterprise Resource Planner (ERP) met all your needs, how long before you could actually get to "All Your Base Are Belong To Us" nirvana? And so we have "Enterprise Application Integration" (EAI) and (new to me...) "Enterprise Information Integration" (EII). One approach to EII in the past has been to build a massive Enterprise Data Warehouse (EDW).
Another approach, Phil Windley writes in his blog item on virtual databases, "The biggest hurdle... is creating the... data model." and includes some useful references.
Another pragmatic source of inspiration is Ralph Kimball. First some history...
A significant contributor to the Xerox Star in the 1970s (which influenced the Mac, which influenced...), Kimball and peers at Metaphor in the 1980s pioneered data integration, iconic capsule programming, and end-user data analysis. Metaphor was sold to IBM and languished under the shadow of OS/2 politics.
In the 1990s Kimball went on to pioneer data warehousing design principles and the Red Brick OLAP database (also sold to IBM via Informix). He is now an independent consultant and popular author on data integration topics for OLAP.
His keys to an effective virtual database (or data bus architecture in his words) is conformed dimensions, smallest grain facts, etc. Most of these principles apply to the largest data warehouses or the smallest databases, and would benefit any Enterprise Information Integration effort.