This has been publicly disclosed by Steven Sinofsky – so there is no NDA disclosure (this sentence has been added for my Microsft’s friends 
).
The next version of Office – currently codenamed Office 15 – will be included on Windows On ARM (WOA).
This looks like a seductive operation in order to make Windows 8 tablets attractive.
By the way, the public beta for Office 15 is planned for this summer (same as previous information )