Hi !
I had the same problem.
Place this code in your functions.php :
add_action( 'user_register', 'so27450945_user_register', 10, 1 );
function so27450945_user_register( $user_id )
{
$user = get_user_by( 'id', $user_id );
$blogname = wp_specialchars_decode( get_option( 'blogname' ), ENT_QUOTES );
$message = sprintf( __( 'New user registration on your site %s:' ), $blogname ) . "\r\n\r\n";
$message .= sprintf( __( 'Username: %s'), $user->user_login ) . "\r\n\r\n";
$message .= sprintf( __( 'E-mail: %s'), $user->user_email ) . "\r\n";
@wp_mail( get_option( 'admin_email' ), sprintf( __( '[%s] New User Registration' ), $blogname ), $message);
}
Normally it should work !
Keep me informed