Someone on foxnews said if they wanted to scare the beejeesus out of us, they'd have a familiar terrorist face giving the warning and not some unknown.
I think too many people assume we know all about how they think. There are lots of reasons they might have used this particular person, such as (1) English fluency, (2) logistics (who was available in the location where the tape was made, (3) perhaps this person was the leader of a particular cell preparing an attack..... etc.
Also, this might be the prelude to a real attack, OR it might be a desperate attempt to influence our election against Bush in the absence of a capacity to carry out an attack on US soil in the next 5 days. It has to be taken very very seriously as an imminent warning, but we also should keep in mind that it might be an attempt to achieve political/propaganda goals without an attack (if they do not have any attacks ready in the US - that's the optimistic scenario).