I’ve written a few posts about how to use CSS3 PIE in WordPress and it’s still evolving (first one, most recent one). What I’m using now is shown below: it’s easy, short, and unlike the other two, has never failed to work on occasion.
/* Conditional styles for old IE */
add_action( 'wp_enqueue_scripts', 'my_load_styles' );
function my_load_styles() {
global $wp_styles;
wp_register_style( 'my-ie8', get_stylesheet_directory_uri() . '/css/ie8.css', false, '0.1', 'all' );
$wp_styles->add_data( 'my-ie8', 'conditional', 'IE 8' );
wp_enqueue_style( 'my-ie8' );
wp_register_style( 'my-ie7', get_stylesheet_directory_uri() . '/css/ie7.css', false, '0.1', 'all' );
$wp_styles->add_data( 'my-ie7', 'conditional', 'IE 7' );
wp_enqueue_style( 'my-ie7' );
}


