In Exchange 2010, you can use Multi-Mailbox Search to search mailboxes for e-Discovery or similar purposes. To preserve data in user mailboxes and protect its integrity, you can place a mailbox on Litigation Hold (also known as legal hold). This prevents mailbox items from being purged from the Store, without impacting user workflow. Additionally, when a mailbox is on litigation hold, a process known as copy-on-write (yes, COW) preserves a copy of the original item when a modification is made to certain item properties.
Note, COW also kicks in for a mailbox when you enable Single Item Recovery - check out RS4's (also known as Ross Smith IV) post Single Item Recovery in Exchange Server 2010 and Understanding Recoverable Items in Exchange 2010 documentation for details.