Basically, UBL wanted to pull a Spain. This tape is an indication that they cannot pull off a major terrorist attack before Nov. 2. The next best thing is to attack with words. He really doesn't understand the American psyche anyway.
It could also be atrial balloon. Just to see which way the electorate leans to see what a real attack would do.
More likely if he is planning an attack, it will be shortly after to bolster the foaming-at-the-mouth Bush-haters into a impeach frenzy that will distract Bush and his admin for a few months.