While I don't see MS porting full office to apple/android, I do see them building a very slick VDI client. Office on a tablet will end up as a vdi session to a private cloud server. It may sound crazy, but its the smart thing to do. It allows Microsoft to leverage all the existing tablets that everyone already has entering the corporate environment. They can support more devices quicker and extend the life of older tablets. The tablets 3 years from now will blow away today's tablets, but if its a VDI client then that wont matter.
Tablets are too personalized and a nightmare for IT security. But what if you could connect to a work desktop and get all your work apps in a way that makes IT feels good about it. Yet, allow the individual to keep personalized apps. I think this is why Windows 8 has such a tablet feel to it. Windows 7 already does a good job under VDI, and I expect Win8 to do so much better.
This would definitely be a corporate IT strategy that is in sync with the MS push of VDI and Private cloud that we see MS timing with the Win8 release. Home users are another story.