Of course it helps Bush tremendously. The American people will rally around the CIC in time of war. UBL attacked Bush by name threatening new attacks. Do we (USA) cave into the craven, mutterings of a twisted zealot by electing a new President? Dobut it.
UBL's threats also places the focus back on the WOT in the closing days of the campaign. That has to help Bush, since he is seen by a large margin to be the best person to wage the WOT. Bush would have won anyway, but this removes all doubt IMO.
The tape reinforces that thought, as you stated correctly.