Ingo summarizes some opinions on message passing versus distributed objects.  It might also be helpful to look at some scenarios.

Those are a bit abstract, I prefer more concrete examples.

"Hello World" is to C as StockQuote is to RPC as PurchaseOrder is to Messaging.

A PurchaseOrder typically consists of some structured data (e.g. address) and a variable number of elements (part numbers and quantities).  Responses (confirmation of shipment) may occur days or weeks later.  Requests are not idempotent.  SMTP might be an appropriate transport for responses.

Recipients of confirmations may want to trigger some actions upon receipt.  There may also be actions that they want to trigger if confirmations are not received in a specified period.  An interesting scenario that captures a number of elements is an RFP/RFQ.

SOAP is an attempt to search for the simplest least common denominator amongst these various scenarios.  Scenarios such as remoting of distributed objects may require additional semantics which can be layered on top of the protocol - and such will require prior and out of band agreement between the participants.

  12:55:29 AM