We recently migrated from 2007 to 2010. However the Manage Site Content and Structure link which points to _layouts/sitemanager.aspx seems to have a look and feel of 2007. How can this be set to use the newer "v4.master" ?
Yes, sitemanager.aspx looks more SP2007-like, but you cannot specify the default master page because it does not use a master page at all. If you look at C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\SiteManager.aspx you will see no master page in the Page directive and no ContentPlaceHolders on the page.