Interesting blog entry by Chris May on the subject.
http://www.chrismay.org/2007/06/14/What+Files+Shoud+Go+In+Source+Control.aspx
Chris links to a Microsoft document: Explained: Structuring Your Solutions and Projects in
Source Control with Team Foundation Server
Summary:
This article explains how you can structure Visual Studio solutions and projects in Team Foundation Server Source Control as well as on your local computer's file-system. The client and server-side folder structures and conventions discussed in this article are not the only approach you can adopt, but they represent an approach proven to work effectively for many team development scenarios working on a variety of different application types.