KB Article: http://support.microsoft.com/kb/2555851
Consider the following scenario. You add the showInAddressBook attribute manually to a mailbox in a Microsoft Exchange Server 2010 Service Pack 1 (SP1) environment. You do this so that the mailbox can appear in certain address lists. You run certain commands on the mailbox, such as the Set-Mailbox command, or the Move-Mailbox command. In this scenario, the mailbox does not appear in certain address lists.