Whoops. Left some pre-release code in there. New version (2.7) available.
Actually, turns out it was a conflict with a feature in the theme my client was using. It had actually been there for months, but no one had stumbled across it. Quick fix to the theme, and everything works fine now.
Thanks.
I think you are looking for wp_get_nav_menus(), defined in wp-includes/nav-menu.php
