As Nikhil Kothari writes in his blog, he's started to use the configSource attribute to split configuration files into smaller pieces.
For example, here is how he exports the profile configuration into a dedicated configuration file.
This is web.config:
<profile configSource="profile.config" />
This is profile.config:
<add name="Name" type="String" />
<add name="Age" type="Int32" />
Make sure you use .config as the extension of your files so they cannot be served to the browser. Avoid .xml for example or your files can be available to prying eyes.
I've been using this feature for my web sites for a while. This allows having a unique web.config file, and separate sub-files for things that are different between the development machine and the hosting environment. For example, I have different connectionStrings.config and smtp.config files for the two environments. The benefits: the web.config file is smaller and hence easier to read, and you don't need a complete web.config file for each environment.