Microsoft has released a tool called the Exchange Server Deployment Assistant. With Office365, one of the options you have is to integrate your on premise Exchange environment with Office 365 so that it operates as online and on-premise act as if they were one mail entity, even though they are two environments. This is what they call the “rich-coexistence” scenario. When setup, you can manage Exchange Online from your on premise Exchange server, use personal archive, and move mailboxes between on premise and online easily.
This is just V1 of the assistant so more details will be forthcoming, but it give a good view into what’s coming.