No matter how lightweight the content, I think one class / interface / etc. per file is essential.
If I'm working on a big solution in Visual Studio I want to be able to see the files and not have to delve inside to see. Even with navigation tools like ReSharper, I want a 1:1 mapping.
If you find a lot of source files with little or no content (maybe extending a class but adding nothing to it) then perhaps you should rethink your design.
