Just wanted to recommend this very interesting Dong's post about WCF and ASP.NET threading in IIS: http://blogs.msdn.com/wenlong/archive/2008/04/21/wcf-request-throttling-and-server-scalability.aspx.
Read the complete post at http://developers.de/blogs/damir_dobric/archive/2008/07/09/wcf-asp-net-request-throttling-and-tuning.aspx