SOA doesn't "require" things like ESB or BPM. It's a more simple idea than that. A plug-in to Sendmail might be one way to implement a SOA.
This Wiki artical defines SOA pretty well: http://en.wikipedia.org/wiki/Service-oriented_architecture
I don't believe physical versus logical "layers" are how the architecture is described.
JBoss's product suite, including JBPM and ESM, can help in executing an SOA. But you can just as easily implement an SOA by providing web services (i.e. SOAP or HTTP) using the basic JBossAS. Those products are just there to help you write a better SOA.