I'm not 100% sure that this is a 3.4 beta bug, but it seems to have started right after updating so it's likely that it is. When I am working with custom taxonomies on the admin end, the corresponding menu item is no longer highlighted in bold like it used to be. For example, if I am on the page for the "Genres" taxonomy for the "Books" custom post type, the "Books" top level menu will be expanded but "Genres" will not be highlighted in bold.
A separate, possibly related, issue I'm seeing is that when I am on the page for any of the taxonomies for the
post post type, the top level menu does not expand at all. But, when I am on the add new or edit posts page, the menu does expand.