I believe it would be a rather massive undertaking.
As is their efforts to re-build office for iOS. But we'll have to wait and see even how full-featured that is.
Office is a product that is heavily tied into the Windows API. So they had a choice to make.. either take everything Office does API wise and create the ARM equivalent, then port over...
Or ground up Office for ARM.
Considering things like
OLE, the fact that Office is also a development environment essentially, etc...
I think ground up Office for ARM that is missing huge chunks of Office's functionality would have been an even bigger effort than just porting the existing underlying pieces of the "Desktop" architecture over to ARM that Office needs.
They could have done that and then hacked it all into the Metro UI somehow.. but it seems like they just decided not too.