Complex Event Processing or Intelligent Event Correlation is not what excites me. That was/is also possible using large data-warehouses and suitable data-mining solutions, but then you learn about it after the fact. It is not so much Complex Event Processing, it is more Complex Event Analysis.
What excites me is the real-time aspect. The possibility to re-act to a set of correlated events as they happen.
The longer I think about it the more use-cases I see. Last month a large (if not the largest) utility provider world-wide visited us in Dublin for a 4-day SOA workshop. Among other things, they are looking for a solution to detect failures in the power grid before they happen or narrow down the location of the failure (faulty transformer) after it happens (e.g. to get a repair team onsite faster).
Last night I found a talk on Google Talks describing a solution for Device-free Passive Localization for Wireless Environments. Another interesting use-case.
CEP is everywhere. I am dying to find out, what the value of a solution for this kind of problems is.