If Phaneuf and Vlasic are capable of stepping in right away, I could see Staal stepping in right away on a team with a weaker group of defenseman than the aforementioned two were up against.
One thing to consider is that the Rangers tend to be pretty conservative with young players. Just about all of the younger guys currently on the team where brought up because of an opening due to injury (they all played well enough to keep their spot after the injured player returned.) So it wouldn't be suprising to see Staal sent to Hartford for at least half of year.
Personally, I think 30-50 games in Hartford is probably prudent. My guess is that he could probably step in immediately next year and not look stupid, but I don't think he's ready to jump in and be a star. Time in the ahl wouldn’t be pointless, instead it might meen that he is a slightly more refined player when he does make the jump. I've heard of quite a few knowledgable people say that the biggest jump is between the junior and the pro ranks, not necessarily ahl to nhl.
At the same time, I've always gotten a sense, from the ranger's brass, that they view Staal differantly than the rest of the organization's prospects, that he's on a differant track. Don Maloney's gone on record to say that sending Marc back to juniors (he was one of the last players cut in camp) was the biggest mistake that organization had made this year. They also signed him immediately after the 05/06 camp, a month and a half after they had picked him. Based on this, I also don't think it would be too shocking that he makes the team right from the start.
He seems like the kind of prospect that could handle either route, he'd end up as a player either way.