If you have had a chance to try out Visual Studio 2008, you likely have seen the rather bulky Web.config file Visual Studio generates when creating a new ASP.NET website or when converting an ASP.NET 2.0 website to 3.5. In one of Scott Guthrie's blog entries a commenter named Mike asked Scott for a rundown of the markup in the default Web.config file generated for version 3.5 ASP.NET applications.
I went ahead and created such an article: Dissecting ASP.NET Version 3.5's Web.config File.
Enjoy!