In this thread at the official Fallout forums, Community Manager Matt "GStaff" Grandstaff dropped the following:
As with Oblivion, when we set out a to release a game, we're looking to
put out the game we want to put out. We let the ESRB make the decisions
on what it should be rated.
That said, no, we probably wouldn't be going for the AO crowd
I believe the talk from Bethesda has always indicated that they were aiming for an M rating, but I suppose we'll have to wait and see what the ESRB says.