Dreamweaver will use the virtual() function to attach included files if you set the "Create Links Relative To" Option on the Local Info tab of your site definition to "Site". The problem with this is that the virtual() function is only valid on some apache servers, and it outputs a bit to the page when the file is attached.
If you set Dreamweaver to use Create Links Relative To the Document, it will use the require_once() function.