In Exchange Server 2010, there’s no more single instance storage (SIS). During development of Exchange 4.0, we had two primary goals in mind, and SIS was borne out of these goals: Ensure that messages were delivered as fast and as efficient as possible; Reduce the amount of disk space required to store messages, as disk capacity was premium. In summary, “Exchange 2010 changes the messaging landscape. The architectural changes we have implemented enable the commoditization of email - providing very large mailboxes at a low cost. Disk capacity is no longer a premium. Disk space is cheap and IT shops can take advantage of larger, cheaper disks to reduce their overall cost. With Exchange 2010 you can deploy a highly available system with a degree of storage efficiency without SIS at a fraction of the cost that was required with previous versions of Exchange.”
More info: Dude, Where's My Single Instance?