I still know the Volturi will interfere somehow. They've become too much of an influence and a threat to just give it up now. I don't think they'll stick with what the Cullens have told them; they'll most likely take matters into their own hands and either kill her or, my theory, turn her. A passage from New Moon:
"'And you, Bella?' Aro raised his eyebrows.
Edward hissed, low in my ears. I stared at Aro blankly. Was he joking? Or was he really asking me if I wanted to stay for dinner?"
Skip forward a couple sentences.
"'Caius, surely you see the potential," Aro chided him affectionately. "I haven't seen a prospective talent so promising since we found Jane and Alec. Can you imagine the possibilities when she is one of us?'"
And forward again:
"'...Besides, I'm so terribly curious to see how Bella turns out!'"
So yeah. It's just something I thought might happen. Even if these statements might have been purely rhetorical. But still; anything is possible at this point.