I'm using W2k AD domain spread acros all country (not so big but ...). And users from AD using samba shares on Centos 4 (single authentication). The best reading howto solve this for me is http://www.brentnorris.net/samba2005.html. It working great form me about 1 year with authenticating users from different locations and using diferent shares on some Centos boxes (main file server).