We've been working quite a bit on documentation that shows the various flavors of cloud hosting. It definitely isn't a one-size-fits-all proposition. For example, some of our customers may want to to move all of their mailboxes to Exchange Online all at once, and others want to move more slowly to cloud-based mail hosting. We have some beta documentation, that I mentioned in a previous post, that discusses a few of these mail-flow scenarios.
One mail-flow scenario of note is the shared-address space with on-premises relay scenario. You might wonder, how do we move a group of our mailboxes to the cloud while maintaining seamless communication with the mailboxes that remain on-premises? If so, the documentation is worth checking out. I think this is going to be a popular configuration.