We have a new article that helps clarify the relationship between ESB and SOA.
As I discussed in IBM says ESBs good, we've added a sidebar to my ESB-Oriented Architecture article. The sidebar confirms, in case there was any doubt, IBM's support for ESBs.
Now another article, "Exploring the Enterprise Service Bus, Part 2: Why the ESB is a fundamental part of SOA," discusses how an ESB fits into an SOA. It's by Greg Flurry and Rachel Reinitz, two of IBM's leading technical spokespeople concerning SOA and ESBs. It's really independent of my article, except that in light of the recent discussion about my article, the authors added a sidebar to discuss my article (and the discussion of my article). Basically, it says that my article is consistent with the points made in this article.
Not only is ESB an important part of any but the most trivial SOA, but also SOA is necessary to achieve value from an ESB. That's why the article talks about using an ESB to expose existing services, creating an ESB that new services then use for connectivity, and making an ESB part of a roadmap for adopting SOA that includes developing services which use the ESB. An ESB is part of an SOA, so they require each other. It's a bit chicken or the egg to specify which comes first, but establishing an ESB should quickly be followed by using the ESB to connect some services, whether the sevices came before or after the ESB.
Like I've now gotten in the habit of telling people: SOA puts the S in ESB.