What expertise is required to implement an ESB?
If you wanted to write the code for an enterprise service bus, you would need to deeply understand distributed computing, messaging, concepts of service-oriented analysis, service-oriented design, service-oriented architecture, service-oriented infrastructure, web services and software security.
Implementations exist that you can use, for example IBM's MQSeries.
MQ Series is not an enterprise service bus. It is software for messaging and queuing. MQ Series can be used to build a proprietary enterprise service bus. However, there is much more to an enterprise service bus than just messaging and queuing.
OpenESB is an implementation of an enterprise service bus.
IBM WebSphere Message Broker is another implementation of an enterprise service bus.
[ July 29, 2008: Message edited by: James Clark ]