Changing the Master Page Association on a Single Web Part Page
SharePoint Designer includes the option to change the master page on individual Web Part Pages and ASPX pages, irrespective of the current site master page. By default, when you create a new Web part page, the Web part page inherits the site’s existing master page, such as v4.master. However, you can choose to change that master page on a page-by-page basis.
Be aware that if you change the master page association on a single page in SharePoint Designer 2010, how the master page is referenced in the content page depends on whether or not the page honors any master page changes via the Web interface. For instance, if you have attached a custom master page to a single content page ...