Microsoft's Exchange Server team announced that the "Cloud Only" option is now available in the latest release of the Exchange Server Deployment Assistant.
"Like the other scenarios in the Deployment Assistant, the "Cloud Only" option allows you to step through a wizard that asks you questions about your current infrastructure and your migration goals. The Deployment Assistant will provide a migration solution that is based on your answers," the Exchange team stated.
More Info: Exchange Server Deployment Assistant
"For many companies, the robust messaging and calendaring features offered by Office 365 for enterprises presents an opportunity to transition all messaging functionality from the on-premises organization into the cloud. Office 365 for enterprises includes a full suite of e-mail migration tools.
However, there are dependencies and restrictions on the migration tools that you may be able to use with Office 365. Variables such as your on-premises Exchange Server version, the number of mailboxes you want to move, and how you want to handle user identity will dictate the tools and processes you can use to migrate to the cloud."