Patrick Leonard nails it rather succinctly. ESB is an EAI pattern (not a product) that may not be suitable for all SOA-related initiatives. 

This doesn't necessarily mean that ESBs are dead, it merely means that there is no single solution for designing and implementing a SOA.   

Silver bullets are only good for guys like Lon Chaney Jr. When it comes to SOA there aren't any silver bullets, regardless of what some vendors or consultants may promise.