Jim Edelen has posted about the process involving CAS and Mailbox server roles to distribute OAB through HTTP. In the Exhcange Server 2007 Beta 2 we have to enable this feature to use because it is not enable by default.
Exchange Server 2007 introduces a new mechanism for distributing Offline Address Books (OAB) that doesn’t require Public Folders. It instead uses HTTP(S) and the Background Intelligent Transfer Service (BITS). There are several potential advantages of the new distribution mechanism including supporting more concurrent clients, reduced bandwidth usage, and more control over the distribution points. It is important to note that the new distribution mechanism requires Outlook 2007, but you can always choose to use both Public Folder Distribution and Web Based Distribution of OABs. That way, older clients can still access their OABs using Public Folders while Outlook 2007 clients can take advantage of the enhanced functionality.
Read more at source: http://msexchangeteam.com/archive/2006/11/15/431502.aspx