In Exchange 2010, we introduced Personal Archives, an email archiving feature intended to allow you to reduce or eliminate PST files by provisioning archive mailboxes for your users. For more details, see Understanding Personal Archives in Exchange 2010 documentation.
In Exchange 2010 RTM, you can provision a user's archive mailbox on the same mailbox database (MDB) as the user's primary mailbox. In Exchange 2010 SP1, you can also create the archive mailbox on a different MDB located on the same Mailbox server as the user's primary mailbox, or on another Mailbox server. Using mailbox move requests, you can also move archive mailboxes independently of the primary mailbox.
With Office 365 beta, we also enabled organizations to store archive mailboxes in the cloud, while maintaining the primary mailbox on your on-premises Mailbox servers running Exchange 2010 SP1. Users can seamlessly access their primary on-premises mailbox and their archive in the cloud using Outlook 2010, Outlook 2007 and Outlook Web App (OWA).
To enable archive in the cloud, your organization must be enrolled in and setup for Office 365 beta (you can sign up for the beta here). (This post is not intended to cover all the rich co-existence scenarios made possible with Office 365 and on-premises Exchange deployments.)
When provisioning an archive in the cloud for a user with a primary mailbox on-premises, the archive mailbox provisioning status in the Exchange Management Console (EMC) changes to Cloud-based archive pending. After the cloud-based archive's created (in Office 365), the archive status is not automatically updated in EMC. Additionally, Outlook and OWA do not detect and load the archive mailbox.
Update 05/11/2011: The archive status is now automatically updated in the EMC and you're not required to use this workaround or the ChangeArchiveStatus.ps1 script.
Here's a workaround to manually update the archive provisioning status in your on-premises organization using the ChangeArchiveStatus.ps1 script (download it from here).
Note: It's a temporary workaround required for the Office 365 beta — it'll not be required for Office 365 GA.
Important: Do not perform the following steps until the archive status in Office 365 changes to active.
Update 1/11/2011: Command to check archive status in cloud updated to Get-MailUser.