The .NET 2.0 and up configuration system is quite powerful and extensible - as long as you don\'t want to change the fact it all comes from XML files in the filesystem.
There's an article here that talks about doing what you are talking about:
http://www.wrox.com/WileyCDA/Section/Redirecting-Configuration-with-a-Custom-Provider.id-291932.html
In summary what they do is create a derived version of ProtectedConfigurationProvider, which is typically used to encrypt .config files. In the Decrypt method, instead of decrypting the configuration information, it's retrieved from a database.