If you start getting template errors (specifically with JoomlaArt templates) like the one below, just replace the lines of code listed below. I've noticed this happen on JA_Fagus and JA_Purity... although I think JA_Purity was fixed in the 1.5.6 Joomla packages.
Fatal error: Call to a member function getItems() on a non-object in /***/templates/ja_***/ja_menus/Base.class.php on line ***.
Open up the file that has the error in it and look for the following line:
$menu = @JMenu :: getInstance();
And replace it with:
$menu = &JSite::getMenu();