Update: Actually, Apple isn't a partner in this initiative, a Microsoft spokesperson confirmed. Microsoft is just building on new iOS 8 app extensions that Apple has made available. These extensions (at least for now) don't allow users to save new documents created in Office to iCloud.
For Office for iPad and iPhone users, Microsoft has added new file-picker integration for viewing and editing. Third-party cloud storage providers can integrate their apps into the "locations" picker in the iOS Office apps, so that users can open, edit and save documents from these services -- not just Microsoft's OneDrive -- from inside of Office.
One of the new locations enabled by this is Apple's iCloud Drive.
Microsoft is working on enabling the same kind of third-party cloud-storage integration for Office universal apps for Windows 10 (meaning the touch-first, Metro-style version of Office for Windows that's currently in preview), as well as for Office for Android apps, according to today's blog post. Microsoft officials didn't provide timing for availability for either of these sets of Office apps.
At the same time, company officials said today, Microsoft also is allowing third-party cloud services to integrate Office Online -- Microsoft's web-ified version of Office -- into their applications. This will allow users of those services to open, view and edit Office Online documents stored in those third-party cloud storage services.
Microsoft is seeking other third-party cloud storage vendors interested in integrating with Office Online to sign up as partners in its Cloud Storage Partner Program. Box, Citrix and Salesforce are already participating in it, today's blog post says.
Surprisingly (not really), Google Drive isn't on the Cloud Storage program list -- at least so far. I'm not holding my breath for that one...