OAB is one of the component that is getting a big boot with Exchange 2007, including the ability to use it without needing public folder (and public folder replication). The change is partially due to the long term plan of Microsoft to get rid of Public Folders and replacing them with Sharepoint as the main collaboration tool. Not to worry though, web download of OAB is only available for now with Outlook 2007 and public folders will be there at least until the next decade.

For more information:

 http://msexchangeteam.com/archive/2006/10/16/429204.aspx