I don't believe it is possible to save the sln file in the website directory. Website Projects are handled differently than Web Applications and as a result have the separate folders.
If you wanted a poor workaround you could create a shortcut to the sln file and put it in the website folder.
To see the differences between Web Applications and Web Site projects check out
http://msdn.microsoft.com/en-us/library/aa730880(VS.80).aspx It doesn't directly answer your question, but the differences between them make it seem like what you are asking is not an option in Visual Studios.