I've researched recommendations about best structure of solutions in Visual Studio, when you have multple projects.
I found the best article "SSW Rules to Better Large Builds in Visual Studio.NET".
See also:
Structuring Solutions and Projects and Managing Dependencies from Microsoft patterns & practices Team Development with Visual Studio® .NET and Visual SourceSafe™ guide.
"Project structure best practices" from searchwebservices.techtarget.com.
Use a Consistent Folder Structure for Solutions and Projects from "Software Configuration and Management Using Visual SourceSafe and VS .NET" article in code-magazine.com
Structuring Solutions in Visual Studio and Team Foundation suggest to have separate solutions for each deployable target.