I don't really want to get in a habit of reposting links to other blogs, but after trying to explain offline address book (OAB) 'stuff' to several people over the last few days I feel it is a good idea to link to an article that explains many of the misconceptions and possible issues with the OAB 

http://www.msexchange.org/articles_tutorials/exchange-server-2010/management-administration/offline-address-book-part1.html 

It is a 3 part series on the OAB a very good read for anyone interested.