I hope this hasn't been posted yet, I couldn't find it anywhere, but the Time magazine poll out last week, which had Kerry up 49% to 43%, also asked: "Would a major terrorist attack on the U.S. before the election make you more likely to vote for Kerry or Bush?" 66% said it wouldn't change their vote, 15% said more likely to vote for Bush, and 16% said they would be more likely to vote for Kerry. As for expectations, rather than personal intentions, 30% said an attack would make a Bush win more likely, while 31% said it would make a Kerry win more likely and 24% said it wouldn't have much effect.
Personally, I think a major attack would help Bush at least a little bit, as he gets to look all "presidential" responding to it, but nevertheless this poll result is good news!