I wonder whether the "routine maintenance" issue that affectedtoday, including Exchange Online, was a symptom of some of the preparatory work that Microsoft needs to do for the forthcoming migration to the Wave 15 set of products.
Related: Office 365 for Everyone - how nice!
You can't expect to make an omelette without breaking some eggs and you can't expect to upgrade a massive multi-tenant environment without some disruption. At least, no one has a recipe to auto-magically transform the Office 365 datacenters so that they're running Exchange 2013, SharePoint 2013, and Lync 2013. It all has to be done through thorough preparation and execution. And in the case of Exchange, by moving every single mailbox to a new server.
Today's outage didn't just affect Exchange as the SharePoint service was also down. I have not seen any reports saying that Lync service was as disrupted.
Some reports state that the problem originated with Windows Azure Active Directory (WAAD), the authentication mechanism used for Office 365. Given the close working relationship that exists between Exchange and Active Directory on-premises or in the cloud, it wouldn't come as a big surprise if service suffered following an Active Directory problem.
In any case, we await with interest the details of the Microsoft post-mortem for the incident. Maybe it will reveal details of their migration plan... or then again, maybe not...
Follow Tony @12Knocksinna