Office 365 : Exchange Online Rich Coexistence
This is really neat. The Exchange Deployment Assistant has been updated to include the migration and configuration requirements to allow rich co-existence between your on-premises Exchange infrastructure and Office 365 – Exchange Online.
Deployment Assistant has been enhanced to include rich coexistence information for those interested in maintaining some users on-premises and some usershosted by Microsoft Office 365 for enterprises in the cloud. This approach is different than the Simple Exchange Migration (SEM) and Staged Migration options currently offered by Office 365. As of now, the available scenario is targeted solely at those organizations with a current Exchange 2003 on-premises implementation, but additional scenarios for supporting Exchange 2007 and Exchange 2010 will be coming. The coexistence information is also only available in English at this time.
Take a look here: Exchange Server 2010 Deployment Assistant.