The monthly fee will only be for the online aspects (mmo, maybe for the dlc of new quests and such as well, if they wanted to be jerks about it) but they clearly stated at the presentation that you can play 100% of the main story without being online. I REALLY don't think they'll force you to buy a $60 game and then pay a monthly fee just to play and finish it.
Not to mention that I really, really doubt we will have to pay outside of Japan for the monthly fee. Monster Hunter Tri is the closest analogy of a Wii game wherein Japanese players had to pay a monthly fee for the online stuff but NA and EU players did not... because NoA and NoE know that such things would never fly here. At least not yet.