I recently ran over the blog of Dave Winer and his post from Sun, Sep 12, 1999 titled "An end to the Über-Operating System". see: http://davenet.scripting.com/1999/09/12/anEndToTheUberoperatingSystem
... The purpose of both specs [SOAP and XML-RPC] is to enable scripted web applications to cross operating system boundaries.
Of course the term AJAX was not known then but I thing that "scripted web applications that cross operating system boundaries" is a good definition of AJAX. By using SOAP there even is more XML on the wire than with AJAX applications using a JSON or plain text transport protocol.
Ask yourself: what is or should be the standard protocol for AJAX applications when retrieving data from the server? - By using SOAP many aspects of a transport layer have been discussed (data types, attachments ...) and there are even more upcoming standard extensions for it (security, routing ...).
The only thing that I really miss is a built-in native SOAP client in browsers.
The (deprecated) SOAP spec from 1999: http://www.oasis-open.org/cover/draft-box-http-soap-00.txt. I think that this doc is one of the most influencing documents I've read. It disappeared from the Microsoft site and http://www.ietf.org. Does anybody know who feels responsible for achieving historical documents from the internet?
Thanks to Don Box, Dave Winer and all the other founders of SOAP.