Stefan Ried, a former SoftwareAG now at Forrester has given an interesting take on possible strategies for Open Source ESBs during a recent Expert Meeting on BPM/BAM/CEP/SOA/EDA.
You can read it from there.
It is interesting to see CEP in the ESB capabilities list for 2008 (as a reminder 2008 is this year). This trend is definitely an interesting one, and folks like WSO2/Synapse have actually already done some prototype work with Esper - as described here on Paul' blog.
You can definitely bet you'll hear much more about those ESBs / CEP integration with Esper throughout this year, despite it is already doable without federating capabilities in one server but using an ESB and an Esper event server bridged together with any kind of transport(JMS f.e.). This has the benefit of keeping the ESB stateless, rather than making it stateful (I consider CEP to be of a stateful kind) but certainly makes the architecture slightly more advanced so I am sure either ways will find their fans.
Open Source Event-Driven SOA is right on Esper' track and WSO2/Synapse has definitely made some good steps forward already despite productization is not yet there right now. Let's wish Mule and JBossESB catch up so as to provide users choices yet consistent CEP semantics - thru Esper of course!