.NET config files (web.config or appname.exe.config) allow us to keep our applications flexible, but we have to remember to manually adjust when deploying to a different environment. On past projects, my solution was to maintain 3 different config files in the project: web.config (dev), web_sit.config, and web_prod.config. This was nice, because when it came time to deploy, you would just copy the appropriate version to web.config, and delete the rest. However, this was NOT nice during development. As you added new settings to the file, you had to make sure you updated all 3 files. They could very easily get out of synch.
A better solution for this is the XmlPreprocess tool. It allows me to use a single file that maintains settings for all environments.