TwentyTen adds some classes to custom menus, for instance:
#access #menu-item-262.current-menu-item, #access #menu-item-262.current-menu-ancestor
All the top-level links in my primary navigation have the class
.current-menu-ancestor when the user is on a child page -- except when I'm viewing content in the custom post type I've built.
How do I set the current page in navigation when viewing a custom post type?