Grady Booch has a nice post up on the difference between SOA and Snake Oil Architecture. I particularlay like this:
Stripped away of all the hype, a Service-Oriented Architecture is essentially a variant of well-proven message-passing architectural patterns. The variance comes in the form that services are cleverly designed to take advantage of the Web-centric infrastructure that pervades many organizations: services allow you to send and receive semantically rich messages through firewalls.
Smalltalk arrived on the message passing frontier a long, long time ago. In a lot of ways, HTTP messaging resembles what happens in Smalltalk - you send the server a message, and if it doesn't understand, it sends you back an appropriate HTTP error message (kind of like a DNU in Smalltalk). The server doesn't crash, it doesn't throw up its hands and stop; rather, it awaits the next message.
This kind of architecture has to be flexible, and growable at runtime. Smalltalk has been that way since the beginning, and HTTP servers operate in much the same way - you can add messages that they'll understand in well understood, dynamic ways. It's kind of nice to see people understanding this strength :)