In Exchange 2010, we introduced the ability for discovery managers to search mailboxes across an Exchange 2010 organization using Multi-Mailbox Search. Along with Personal Archives, which allow you to eliminate the risk from PST files and store historical messaging data in Exchange, Legal Hold, which allows you to preserve data for eDiscovery, and Retention Policies, which allow you to manage email lifecycle, it has been one of the more popular features of Exchange 2010.
Multi-Mailbox Search required an Enterprise Client Access License (CAL) for each mailbox searched. We’ve heard your feedback on how you use this feature and the licensing requirements. In the coming months, we’re making a change to Exchange 2010 licensing so you’ll no longer require an Enterprise CAL for Multi-Mailbox Search. The Exchange Server Licensing page will be updated to reflect this.
7/16/2012: We've updated the Exchange Server Licensing page.
7/27/2012: This blog has been updated to reflect the timing of this change. See Update on Multi-Mailbox Search Licensing for details.