Actually Not Disruptive
I can't entirely see why this would piss Microsoft off. In the corporate world if people use this they're paying for Google Apps, but not using the web interface (which Google harp on about forever and a day) and still paying the license fee to Microsoft. Both companies make money from this arrangement. Also, as a user is likely to upload to Google apps through the Office client, and Download through the Office client, why will they ever need to use the Google interface? They wont. They'll just see Google Apps as this dumb box in the cloud to store their documents. Which is exactly what Microsoft Office Live is which comes free as of June/July when Office 2010 comes out.
Really can't see the disruptive angle Google are pitching with this one, and can only see it fading away in a couple of months when built into office is the same feature but going into a Microsoft server/internal sharepoint site rather than Google Apps.