SOA Testing has really become a mainstream discipline. Better Software Magazine, a publication for QA Professionals and anyone focused on Quality, published a coverpage article on Building and Testing your first Web Service.
This article will get you started on SOA Testing in no time and provide you a good overview on how web services are built and how they can be tested. It's great article to get your feet wet with SOA and SOA Testing.
You can read the full article here.