functions.php

<?php

add_action( 'after_setup_theme', 'bootstrap_setup' );

if ( ! function_exists( 'bootstrap_setup' ) ):

	function bootstrap_setup(){

		add_action( 'init', 'register_menu' );
			
		function register_menu(){
			register_nav_menu( 'top-bar', 'Bootstrap Top Menu' ); 
		}

		class Bootstrap_Walker_Nav_Menu extends Walker_Nav_Menu {

		      /* included params: $depth = 0 and $args = array(), 
		      otherwise a "Strict Standards" error comes up */
                        function start_lvl( &$output, $depth = 0, $args = array() ) {

				$indent = str_repeat( "\t", $depth );
				$output	   .= "\n$indent<ul class=\"dropdown-menu\">\n";
				
			}

			function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
				
				$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

				$li_attributes = '';
				$class_names = $value = '';

				$classes = empty( $item->classes ) ? array() : (array) $item->classes;
				$classes[] = ($args->has_children) ? 'dropdown' : '';
				$classes[] = ($item->current || $item->current_item_ancestor) ? 'active' : '';
				$classes[] = 'menu-item-' . $item->ID;


				$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );
				$class_names = ' class="' . esc_attr( $class_names ) . '"';

				$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
				$id = strlen( $id ) ? ' id="' . esc_attr( $id ) . '"' : '';

				$output .= $indent . '<li' . $id . $value . $class_names . $li_attributes . '>';

				$attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
				$attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
				$attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
				$attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
				$attributes .= ($args->has_children) 	    ? ' class="dropdown-toggle" data-toggle="dropdown"' : '';

				$item_output = $args->before;
				$item_output .= '<a'. $attributes .'>';
				$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
				$item_output .= ($args->has_children) ? ' <b class="caret"></b></a>' : '</a>';
				$item_output .= $args->after;

				$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
			}

			function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {
				
				if ( !$element )
					return;
				
				$id_field = $this->db_fields['id'];

				//display this element
				if ( is_array( $args[0] ) ) 
					$args[0]['has_children'] = ! empty( $children_elements[$element->$id_field] );
				else if ( is_object( $args[0] ) ) 
					$args[0]->has_children = ! empty( $children_elements[$element->$id_field] ); 
				$cb_args = array_merge( array(&$output, $element, $depth), $args);
				call_user_func_array(array(&$this, 'start_el'), $cb_args);

				$id = $element->$id_field;

				// descend only when the depth is right and there are childrens for this element
				if ( ($max_depth == 0 || $max_depth > $depth+1 ) && isset( $children_elements[$id]) ) {

					foreach( $children_elements[ $id ] as $child ){

						if ( !isset($newlevel) ) {
							$newlevel = true;
							//start the child delimiter
							$cb_args = array_merge( array(&$output, $depth), $args);
							call_user_func_array(array(&$this, 'start_lvl'), $cb_args);
						}
						$this->display_element( $child, $children_elements, $max_depth, $depth + 1, $args, $output );
					}
						unset( $children_elements[ $id ] );
				}

				if ( isset($newlevel) && $newlevel ){
					//end the child delimiter
					$cb_args = array_merge( array(&$output, $depth), $args);
					call_user_func_array(array(&$this, 'end_lvl'), $cb_args);
				}

				//end this element
				$cb_args = array_merge( array(&$output, $element, $depth), $args);
				call_user_func_array(array(&$this, 'end_el'), $cb_args);
				
			}
			
		}

	}

endif;
?>

header.php

<div class="navbar navbar-fixed-top">
	<div class="navbar-inner">
		<div class="container">
			<?php
				
				$args = array(
					'theme_location' => 'top-bar',
					'depth'		 => 2,
					'container'	 => false,
					'menu_class'	 => 'nav',
					'walker'	 => new Bootstrap_Walker_Nav_Menu()
				);

				wp_nav_menu($args);
			
			?>
		</div>
	</div>
</div>