Archive mailboxes are a new concept in Exchange 2010 that solve some of the problems inherent in PST archives – the primary of which being that there’s no way to access your PST files from a different machine. Archive mailboxes solve this by providing you a second mailbox you can use to archive mail from your primary mailbox in a location that is accessible via OWA and from other computers.
In a future release, there should be functionality to access these archive mailboxes with web services, but as of the time of this writing, there’s no way to access the archive mailboxes with EWS.
You can, however access these new mailboxes with MAPI. You simply pass the legacy DN of the mailbox into IExchangeManageStore::CreateStoreEntryId to create an ENTRYID you can use in your call to OpenMsgStore. The LegacyDN you need can be retrieved from a call to the new SOAP AutoDiscover service. Simply call GetUserSettings and include the AlternateMailboxes setting in your RequestedSettings value. If you get a result in this property, and it looks just like your UserDN but with an added /guid=<some guid> at the end, then that’s your archive mailbox.