Master page editing has been disabled for this site—SharePoint Designer

By default in SharePoint Foundation 2010, site owners and designers don’t have access to edit the master pages for sites.  You get the following error when you try to open the site in SharePoint Designer 2010 and try to edit the master page:

“Master page editing has been disabled for this site.  Contact the site collection administrator to request permission.”

In order to allow this, a site collection administrator should go to the top level site settings by going to Site Actions –> Site Settings and clicking on the SharePoint Designer Settings link under Site Collection Administration:

Within these settings there is an option for Enable Customizing Master Pages among other settings: