Create new category upon save based on post information

First, some comments on your code:

• the globals $post and $wpdb are not needed, in any case you can declare them as global $post, $wpdb;
• I don’t understand your use of this conditional if (is_null($imgID))
• you’re using get_term_by with “slug”, but I guess it’s better to do it by “name”

The following is a working code, but I’m not sure if there are any flaws in it. As I was not able to debug the save_post using FirePHP, I did a dirty trick dumping the vars inside a custom field: update_post_meta($post_ID, 'cf_debug_info', print_r($imgmeta,true));.

add_action('save_post', 'wpse_53549_add_post_camera_category', 10, 2);

function wpse_53549_add_post_camera_category($post_ID, $post) {

    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;

    $images = get_children( array(
        'post_parent' => $post_ID,
        'post_type' => 'attachment',
        'numberposts' => 1,
        'post_mime_type' => 'image',
        'orderby' => 'ID',
        'order' => 'ASC'
    ));

    if ( $images ) 
    {
        $image = array_shift( $images );
        $imgID = $image->ID;
    }

    if ( !$imgID ) 
            return;

    $imgmeta = wp_get_attachment_metadata($imgID);

    if ( $imgmeta )
    {
        // this info may be useful when checking for has_term
        // $defaultcat = get_option('default_category');

        $camera = ($imgmeta['image_meta']['camera']) ? $imgmeta['image_meta']['camera'] : false;        

        if ( $camera )

            // Add to custom category
            if( !has_term( $camera, 'category', $post_ID ) )
            {
                $category = get_term_by( 'name', $camera, 'category' );
                if ( $category )
                {
                    $cat = array( $category->slug );
                    // the last parameter sets if the term must be appended or overwrite the previous
                    wp_set_object_terms( $post_ID, $cat, 'category', true );
                }
                else
                {
                    $id = wp_create_category( $camera );
                    $new_cat = get_term_by( 'id', $id, 'category' );
                    wp_set_object_terms( $post_ID, $new_cat->slug, 'category', true );
                }
            }

    }

}