Here is an article with some custom rules and plug-ins that you can use to apply dynamic querystring correlation to your web tests.  These can be useful if you are testing a site that uses querystring parameters to track dynamic values (such as session id), for example, a SQL Reporting Services site.

http://blogs.msdn.com/densto/pages/dynamic-querystring-correlation-custom-extraction-rules-and-webtest-plug-in.aspx