Sergei Baranov's article in Sys-Con WebSphere Journal gives a good overview of what's going on for Web Service performance testing in practice.
His article provides strategies for creating load test scenarios that reflect tendencies of the real world environment. To help us apply these strategies, it introduces best practices for organizing and executing load tests, and suggests how these best practices fit into a Web services application's development life-cycle.
It also demonstrates how to utilize performance monitors offered by WebSphere and the operating system, as well as how to analyze load testing results. The bulk of the discussion assumes that WSDL is used to describe the Web service's interface, and that SOAP and HTTP are used for the messaging and transport layers, respectively.