Olvide mencionar tambien que he probado varios themes para asegurarme de que no era el estilo que estaba mal hecho y pasa lo mismo con los 3 que he probado...Aquí tienes el código del header.php
    
Código PHP:
Ver original- <?php 
- /** 
-  * The Header for our theme. 
-  * 
-  * Displays all of the <head> section and everything up till <div id="main"> 
-  * 
-  * @package WordPress 
-  * @subpackage Twenty_Ten 
-  * @since Twenty Ten 1.0 
-  */ 
- ?><!DOCTYPE html> 
- <html <?php language_attributes(); ?>> 
- <head> 
- <meta charset="<?php bloginfo( 'charset' ); ?>" /> 
- <title><?php 
-     /* 
-      * Print the <title> tag based on what is being viewed. 
-      */ 
-     global $page, $paged; 
-   
-     wp_title( '|', true, 'right' ); 
-   
-     // Add the blog name. 
-     bloginfo( 'name' ); 
-   
-     // Add the blog description for the home/front page. 
-     $site_description = get_bloginfo( 'description', 'display' ); 
-     if ( $site_description && ( is_home() || is_front_page() ) ) 
-         echo " | $site_description"; 
-   
-     // Add a page number if necessary: 
-     if ( $paged >= 2 || $page >= 2 ) 
-         echo ' | ' . sprintf(-  __ ( 'Page %s', 'twentyten' ), max( $paged, $page ) );
 
-   
-     ?></title> 
- <link rel="profile" href="http://gmpg.org/xfn/11" /> 
- <link rel="stylesheet" type="text/css" media="all" href="<?php bloginfo( 'stylesheet_url' ); ?>" /> 
- <link rel="pingback" href="<?php bloginfo( 'pingback_url' ); ?>" /> 
- <?php 
-     /* We add some JavaScript to pages with the comment form 
-      * to support sites with threaded comments (when in use). 
-      */ 
-     if ( is_singular() && get_option( 'thread_comments' ) ) 
-         wp_enqueue_script( 'comment-reply' ); 
-   
-     /* Always have wp_head() just before the closing </head> 
-      * tag of your theme, or you will break many plugins, which 
-      * generally use this hook to add elements to <head> such 
-      * as styles, scripts, and meta tags. 
-      */ 
-     wp_head(); 
- ?> 
- </head> 
-   
- <body <?php body_class(); ?>> 
- <div id="wrapper" class="hfeed"> 
-     <div id="header"> 
-         <div id="masthead"> 
-             <div id="branding" role="banner"> 
-                 <?php $heading_tag = ( is_home() || is_front_page() ) ? 'h1' : 'div'; ?> 
-                 <<?php echo $heading_tag; ?> id="site-title"> 
-                     <span> 
-                         <a href="<?php echo home_url( '/' ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a> 
-                     </span> 
-                 </<?php echo $heading_tag; ?>> 
-                 <div id="site-description"><?php bloginfo( 'description' ); ?></div> 
-   
-                 <?php 
-                     // Check if this is a post or page, if it has a thumbnail, and if it's a big one 
-                     if ( is_singular() && current_theme_supports( 'post-thumbnails' ) && 
-                             has_post_thumbnail( $post->ID ) && 
-                             ( /* $src, $width, $height */ $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'post-thumbnail' ) ) && 
-                             $image[1] >= HEADER_IMAGE_WIDTH ) : 
-                         // Houston, we have a new header image! 
-                         echo get_the_post_thumbnail( $post->ID ); 
-                     elseif ( get_header_image() ) : ?> 
-                         <img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" /> 
-                     <?php endif; ?> 
-             </div><!-- #branding --> 
-   
-             <div id="access" role="navigation"> 
-               <?php /*  Allow screen readers / text browsers to skip the navigation menu and get right to the good stuff */ ?> 
-                 <div class="skip-link screen-reader-text"><a href="#content" title="<?php esc_attr_e( 'Skip to content', 'twentyten' ); ?>"><?php _e( 'Skip to content', 'twentyten' ); ?></a></div> 
-                 <?php /* Our navigation menu.  If one isn't filled out, wp_nav_menu falls back to wp_page_menu.  The menu assiged to the primary position is the one used.  If none is assigned, the menu with the lowest ID is used.  */ ?> 
-                 <?php-  wp_nav_menu ( array( 'container_class' => 'menu-header', 'theme_location' => 'primary' ) ); ?>
 
-             </div><!-- #access --> 
-         </div><!-- #masthead --> 
-     </div><!-- #header --> 
-   
-     <div id="main"> 
Gracias por responder :)