My solution in Visual Studio 2012 currently contains two projects:
Both, the DLL an
You better abstract the use of your logging mechanism. I described this in this blog post, it's about log4net but is the same principle whatever framework you use. In any case, you need the log assembly in every project where you use it, but by abstracting it it's easy to replace it by something else (when testing for example). Logging is infrastructure, so you would put the interfaces and concrete implementation in an infrastructure project, and reference that one from the projects in which you want to log.