I think Nick's pretty correct in his assessment. It's hard to deny that we're dealing with significant blowback. However, you run that risk through action or inaction. Sometimes you're just not going to win, and insofar as pissing off the lunatic element, sometimes you're not going to avoid it. If it weren't Bin Laden, it could just as easily been some other asshole with some other list of grievances; merit is irrelevant.
Let's also keep in mind that Bin Laden isn't necessarily the most reliable source of intel regarding his intentions. For one thing, there's a difference between the official story, the story you use to motivate your followers, and the story that really speaks to you. Furthermore, I'm sure he was fairly pissed off about our presence in the Holy Land, but I think there was probably more to it than that. It seems to me that the guy was, for lack of a better term, a natural born mujahideen, and he was going to follow his path, regardless of the who or why.
So in short, yes our foreign policy played a part in the attacks, but it probably didn't make any difference one way or the other.