This is something that came up the other day that I thought was a valuable topic. So here is the brief synopsis.
Scope is usually smaller. A Department has a specific series of questions to answer by implementing a solution and typically have a readily available infrastructure.
In an enterprise model you have stake holders from multiple buisness units that all want something specific or from such a high level the scale of the implementation and integration takes a significantly larger amount of time.
Suddenly getting the data is more complex and even though having a stake holder at a high level, it's not a priority of the day to day.
So how do you successfully implement a BI Project at the Enterprise Level thats successful?
Well it depends on the departments contributing, and scope of the project. Top down doesn't always work in this case, start with the business units and establish a consistant tool set.
Don't hesitate to focus on a business Unit that is more cooporative than another, as soon as you show results and demonstrate value with the companies data and not an example, the rest of the pieces start to fall into place.
You can then start to put dates around the completion of the integration of each business units piece and as this happens the visibility and value to the stake holders increases.