I agree that this particular joke wasn't very funny, so it moves into the realm of tasteless. However, if somebody comes up with a legitimately funny, laugh out loud, terrorist/al qaeda joke, more power to them.
The February after 9/11, I remember somebody made this Valentine's Card that portrayed the twin towers with smily faces, and it said I'm Falling For You. I laughed so hard at that. I felt guilty, but humor doesn't really recognize tact sometimes.