<p>Why are the code-behind pages for an ASP.NET web application locked at run time? I have older projects (probably defined as "web sites" instead of "web apps") where I can edit the code behind, refresh the browser, and see my changes. With the web app, I have to continually close and reopen the browser if I want to see my changes live. Is there a setting or something I'm missing to allow me to edit at run time, and without restarting the debugging session?</p>

When using IIS as your development server, Edit and continue is not currently available for Visual Studio for ASP.NET.
See this
blog entry
, however, that shows it is possible for the
Visual Studio 2005 Web Application Project
Project type. It requires using the Visual Studio Development Server (Cassini) during development instead of your local IIS (see the properties box on the project).