Issue of KM ROI in Organizations

One of the major issues Knowledge Management practice in an organization face today are the constant requirement to convince the Management that they are creating value. They are constantly faced with the issue of showing the so called ROI. Of course looking for ROI in any investment is a very good thing but what should be remembered is that ROI in systems like KM cannot be clearly quantified and takes a longer time as its scope extends to the entire organizations and cannot be classified as only front-end or back-end systems. I would love to hear about KM ROI models in various organizations. Ideally if the KM processes are well defined and systems in place ROI calculations should be pretty direct atleast to some extent but as KM processes definiton in organizations vary from each other, it is not very easy to replicate a model of one organization in another.