A relatively recent posting from MSFT. In this posting there is a question about the relationship between the batch oriented Map Reduce, and the on-line oriented event processing. The answer, according to MSFT is - event processing can be used as a reducer in the Map Reduce, where there are multiple copies of an event processing engine perform the reduce function.
I have written before about offline event processing, with the insight that event processing is useful not only in online, but in offline, since it provides both efficient implementation and high-level abstraction in certain functions (pattern matching, aggregation and more) that makes it also attractive to use in batch.
Of course, another synergy may be using event processing within real-time hadoop, like Darkstar. as is frequently articulated by Colin Clark.