The esb is pretty well packaged, being the jbossesb.sar and a bunch of .esb service archives, which should make installing or upgrading pretty straightforward.
Things for a bit hairy when the esb needed newer features/products then jbossas ships, such as JBossMessaging and a new JBossWS implementation. This is precisely the problem the jbossesb server takes solves, and it gives you a ready-to-go, ou-of-the-box platform. JBossESB will probably always have slightly different requirements, then JBossAS.
The current JBossESB server is based on jbossas-4.2.1.GA. Apart from the differences mentioned above it should administrate and configure the same.
The JBossESB server platform is one of the solutions for which we offer support if you want to take it to production.
My preference would be so see the JBossESB as a platform of it's own for the ESB core services, and not to mix it with your applications build on JBossAS, as you probably have different uptime requirements for the two, but this entirely up to what your production requirements look like. I would then host my own business specific services on the appserver where the business logic is located. The ESB becomes a truly distributed pieces of software this way.