An excellent example of this is Mathematica http://www.wolfram.com/products/mathematica/index.html. Regions of the page are code, regions are interactive, and regions are presentation output.
In Visual Studio we could use this concept to include unit test results or generated HTML documentation. Imagine if you could have an interactive grid inline in the editor that you could use to test functionality of the method you're working on.
Obviously there are plenty of times when this kind of bloat is undesirable, but I think it could be fantastic where you need code to be highly documented.
In terms of the source, it could probably be added into the existing triple-slash XML comment system. Visual Studio could optionally render these features (allow it to be switched off for performance), other editors would see comments only.