MomentumSI's Definition of the ESB:
"An ESB is a collection of platform specific (Java) API's front-ending a vast subset of the Web service protocols, formats and identifiers."
I almost forgot - I've been meaning to congratulate Sonic on inventing 'SOAP over JMS', also known as the ESB. Unofficially, MomentumSI has been predicting that the bus would crash since October of 2003.
Here is the really sad news: ESB isn't dead as a category. It won't die until a replacement category is created. In my humble opinion, the ESB 2.0 won't work. What is the replacement: The Enterprise Service Network? Application Oriented Networking? I don't know. What I do know is that in an unprecedented display of incompetence Gartner & Sonic took a shotgun to the ESB category and blew it away.
Annrai, Frank, Dan, Roman - uhh... what the hell are you guys waiting for??? I'm going to start referring to you guys as the "Ball-less-four".