<?php
// Create a dynamic id on body elements
function get_body_id( $id = '' ) {
    global $wp_query;

    // Fallbacks
    if ( is_front_page() )  $id = 'front-page';
    if ( is_home() )        $id = 'blog';
    if ( is_search() )      $id = 'search';
    if ( is_404() )         $id = 'error404';
        
    // If it's an Archive Page
    if ( is_archive() ) {
        if ( is_author() ) {
            $author = $wp_query->get_queried_object();
            $id = 'archive-author-' . sanitize_html_class( $author->user_nicename , $author->ID );
        } elseif ( is_category() ) {
            $cat = $wp_query->get_queried_object();
            $id = 'archive-category-' . sanitize_html_class( $cat->slug, $cat->cat_ID );
        } elseif ( is_date() ) {
                if ( is_day() ) {
                    $date = get_the_time('F jS Y');
                    $id = 'archive-day-' . str_replace(' ', '-', strtolower($date) );
                } elseif ( is_month() ) {
                    $date = get_the_time('F Y');
                    $id = 'date-' . str_replace(' ', '-', strtolower($date) );   
                } elseif ( is_year() ) {
                    $date = get_the_time('Y');
                    $id = 'date-' . strtolower($date);
                } else {
                    $id = 'archive-date';
                }
        } elseif ( is_tag() ) {
            $tags = $wp_query->get_queried_object();
            $id = 'archive-tag-' . sanitize_html_class( $tags->slug, $tags->term_id );
        } else {
            $id = 'archive';
        }
    }
    
    // If it's a Single Post
    if ( is_single() ) {
        if ( is_attachment() ) {
            $id = 'attachment-'.$wp_query->queried_object->post_name;
        } else {
            $id = 'single-'.$wp_query->queried_object->post_name;
        }
    }

    // If it's a Page
    if ( is_page() ) {
        $id = 'page-'.$wp_query->queried_object->post_name;
        if ('' == $id ) {
            $id = 'page';
        }
    }
    
    // If $id still doesn't have a value, attempt to assign it the Page's name
    if ('' == $id ) {
        $id = $wp_query->queried_object->post_name;
    }
    
    $id = preg_replace("#\s+#", " ", $id);
    $id = str_replace(' ', '-', strtolower($id) );
    
    // Let other plugins modify the function
    return apply_filters( 'body_id', $id );    

};

// Print id on body elements
function body_id( $id = '' ) {
    if ( '' == $id ) {
        $id = get_body_id();
    }
    
    $id = preg_replace("#\s+#", " ", $id);
    $id = str_replace(' ', '-', strtolower($id) );

    echo ( '' != $id ) ? 'id="'.$id. '"': '' ;
};
?>

http://darrinb.com/notes/2010/adding-a-custom-id-to-the-body-element-in-wordpress/

Add a custom ID to the body, useful for when the classes added to the body aren't descriptive enough on pages like Archives and single

Add a Custom ID to the Body Element in WordPress

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