The licensing for Exchange 2010 has been changed to allow multi-mailbox search with standard CALs instead of enterprise CALs. Previous to this change any mailbox searched with multi-mailbox search required an enterprise CAL.
This is a pretty big deal for a few reasons. First, it's handy to use multi-mailbox search when recovering items for single item recovery. Much nicer interface than straight PowerShell. Secondly, it's also nice when users complains that they've lost a message and you can go a search their mailbox without connecting directly to their mailbox.
You still need an enterprise CAL for personal archives, legal hold, per user/group journaling, and custom retention policies.
The Exchange 2010 licensing page (http://www.microsoft.com/exchange/en-us/licensing-exchange-server-email.aspx#Exchange2010CAL) has been updated to show the change.