The beta for Visual Studio 2008 SP1 is now available and includes enhancements to both the Web test and load test APIs that are described in these articles:

http://blogs.msdn.com/billbar/pages/web-test-api-enhancements-available-in-vsts-2008-sp1-beta.aspx

http://blogs.msdn.com/billbar/pages/load-test-api-enhancements-in-vsts-2008-sp1-beta.aspx