Did you know that SOAPbox includes a command-line driven utility for stress-testing a Web Service? No? Grab your copy on the
Vordel SOA product page here. You can simulate multiple clients using parallel threads, send a stream of messages over time, include waits, and when it's done it generates reporting info. Part of the online help (which you also get with SOAPbox) is shown in the screenshot below, indicating some of the functionality you get with the command-line testing.