We wanted to let you know about a problem that can cause some or all of the replicas of a public folder to unexpectedly disappear when changing the replica lists on Exchange 2010. This can happen when using the Exchange 2010 RTM (Released to Manufacturing) Public Folder Management Console, the Set-PublicFolder CMDlet (or scripts that call it) or the ExFolders tool connected to Exchange 2010 RTM server. If only some of the replicas disappear, you may try to add them back, only to see them instantly vanish again. If all of the replicas disappear, you must restore the public folder data from a backup.
This is due to a bug in the Information Store in Exchange 2010 RTM. When Store receives the new replica list from the admin tools, under certain conditions it makes the wrong decision about which replicas are supposed to be added and which are supposed to be removed. This bug has been fixed in Exchange 2010 Service Pack 1 (SP1) Store.
This behavior will only affect folders that were originally created when Exchange 5.5 was present in the environment, so you typically only see this on very old public folders
Read more at source: http://msexchangeteam.com/archive/2010/10/04/456502.aspx