InfoWorld has a special report:
The golden dream behind the ESB is to replace proprietary integration brokers with open communication layers through which distributed services and business processes are readily exposed and easily managed. The immediate reality, however, is that it may be too soon to leave the old messaging subsystems behind.
Regardless of the underlying messaging core, an ESB must somehow — through open standards or by proprietary means — create a foundation for reliable messaging. Until WS-* specifications for reliable messaging fall into place, that reliability continues to come from the likes of JMS (Java Message Service), homegrown messaging engines, proprietary MOM (message-oriented middleware), and J2EE servers.