I have written a number of white papers and sample applications about Web services integration with Application Express and they are now hosted on their very own page on OTN. Check it out here.
My colleague, Sathish Kumar, wrote the YouTube sample application. It was his work with that application which led to the discovery that the Manual Web references feature of Application Express works with XML-RPC style Web services and inspired the white paper with the same title.
Also, while we are talking about Web services and Application Express, check out Tyler's post on integrating Application Express with BiPublisher through the same Manual Web references feature of Application Express. It is a very nice and useful example.