The latest version (3.3) of the WSTK on IBM alphaWorks (see <http://www.alphaworks.ibm.com/tech/webservicestoolkit/>) includes some performance testing tools called the "WSST". Thanks to the WSTK deployment on developerworks, you can see the WSST documentation at <http://dwdemos.alphaworks.ibm.com/wstk/common/wstkdoc/services/demos/wsst/client/doc/index.html>. The WSST are very modular and flexible; you will find they give you more control over the simulated client population than most tools. I have been using the WSST for stressing web services in carefully callibrated ways.
Mike
