Actually, it probably makes more sense to move it into admin_init() and get rid of the is_admin() check.
I was able to fix this by moving lines 18-21 of ihaf.php into the init() function.
function init() {
load_plugin_textdomain( 'insert-headers-and-footers', false, dirname( plugin_basename ( __FILE__ ) ).'/lang' );
if (is_admin()) {
wp_register_style('IHAFStyleSheet', IHAFURL . '/ihaf.css');
wp_enqueue_style( 'IHAFStyleSheet');
}
}
