The web applications I develop often require co-dependant configuration settings and there are also settings that have to change as we move between each of our environments.
The custom configuration are quite handy thing and often applications end up with a demand for an extendable solution.
For .NET 1.1 please refer the article http://aspnet.4guysfromrolla.com/articles/020707-1.aspx
Note: The above solution works for .NET 2.0 as well.
For .NET 2.0 specific solution, please refer the article http://aspnet.4guysfromrolla.com/articles/032807-1.aspx