I was just going through this presentation (which is scarily thorough concerning Middleware), when I came across the following quotes by Ken Klingenstein, Chief Technologist at the University of Colorado at Boulder and a Director of Internet2 Middleware Initiative:
Middleware makes "Transparently use" happen
Middleware is the intersection of what the Network Engineers and the Application Programmers don’t want to do
I think of Middleware as the complexity sink of information and the continuous process of reducing the inherent entropy therein (i.e. increasing the manageability of it).