I have a project called Data which is a data layer. In this project, all files are just lying in the top folder. I have enumerations, POCOs, repositories, partial classes and so
I (currently - changes based on project) tend to use this approach when naming assemblies/projects/namespaces in a SAAS/Web style project)
To explain the Services/Service Client...I use an IoC (currently StructureMap) that allows my WebClient to either speak directly to the Business layer or to be redirected to speak through the ServiceClient through Services to the Business layer. This gives me the flexibility to either deploy my app layer to my web applications or to distribute pieces of my business layer (app layer) to different servers by way of WCF/SOA principles.