According to MOSS 2007 document, master page and page layout are working together as a template to control content pages' look, feel and content. In top site, there is a "Master page and Page Layout Gallery" and you could find lots out-of-box master and page layout (aspx) file there.
But I am frustrating in finding the relationship between master page and the page layout. It's able to create/upload many master pages or create many page layout, but I found all new page layout use default.master as master page and couldn't find a way to associate to other master page.
Does anybody know the answer?