It's a good move for Microsoft, but it's not entirely obvious if this is a good move for dropbox or the storage ecosystem.

For example, Box is also working with Microsoft: http://content.box.com/box-for-office-365 -- I would expect integrations with other platforms in the future as well. If the consumer experience is driven around Office apps rather than around the Dropbox app, then at one point the cloud storage solutions truly become interchangeable.