I followed the general instructions on this page
http://www.dokuwiki.org/auth:ad and am now using the following configuration (actual names have been changed):
$conf['useacl'] = 1;
$conf['disableactions'] = 'register';
$conf['autopasswd'] = 0;
$conf['authtype'] = 'ad';
$conf['passcrypt'] = 'ssha';
$conf['auth']['ad']['account_suffix'] = '@company.com';
$conf['auth']['ad']['base_dn'] = 'DC=company,DC=com';
$conf['auth']['ad']['domain_controllers'] = 'dc1.company.com,dc2.company.com';
$conf['superuser'] = '@ad_support_group';
We're running CentOS 4.7 and AD 2008.