add_action('wp_head', 'artStyle');

function artStyle(){
     global $post;
     if(is_single()) {
          $currentID = $post->ID;
          if(file_exists(TEMPLATEPATH.'/style-'.$currentID.'.css')) {
               echo '<link rel="stylesheet" href="'.get_bloginfo('template_url').'/style-'.$currentID.'.css'.'" type="text/css" media="screen" />'."\n";
          }
     }
}

I borrowed some stuff form my ie_stylesheet function which looks for ie stylesheets in current theme and include them. Doesn’t hurt to copy that here to, its very helpful:

function ie_stylesheet(){
     if(file_exists(TEMPLATEPATH.'/ie.css')){
          echo '<!--[if IE]><link rel="stylesheet" type="text/css" media="screen, projection" href="'.get_bloginfo('template_url').'/ie.css'.'" /><![endif]-->';
          echo "\n";
     }
     if(file_exists(TEMPLATEPATH.'/ie6.css')){
          echo '<!--[if lte IE 6]><link rel="stylesheet" type="text/css" media="screen, projection" href="'.get_bloginfo('template_url').'/ie6.css'.'" /><![endif]-->';
          echo "\n";
     }
     if(file_exists(TEMPLATEPATH.'/ie7.css')){
          echo '<!--[if gte IE 7]><link rel="stylesheet" type="text/css" media="screen, projection" href="'.get_bloginfo('template_url').'/ie7.css'.'" /><![endif]-->';
          echo "\n";
     }
}