Traditionally, Identity and Access Management has been a very large complex implementation that required a CIO to choose a platform for the duration of the entire implementation. You may find choose from some very good products like Novell identity Manager, Oracle Identity Manager, CA Identity Manager, and may others. This served its purpose at the time, and the benefits were well documented.
Recently, with the new economy and the evolution of business systems and requirements, this strategy, we will refer to as the "Big Bang" IDM strategy has, in many cases created a "living project". A Living Project is one that never really seems to find its completion. That is where scale out IDM comes in.
Scale Out IDM is a strategy where Identity practices are put in play to allow for a more agile and flexible deployment, that is more modular and purposed. It may or may not include more than one platform, and has with it some best practices such as SOA, Identity Integration and Consolidation, and the removal of mixed dependencies that come with Point to Point / Hub and Spoke Models.