Let me start by saying that I think Carlin was one of the funniest, cleverest people around, and one of the smartest comedy writers around, with an uncanny ear for rhythm; and furthermore I have agreed with his political, religious, and social stances as portrayed in his comedy.
But this was apparently Carlin's first HBO special after 9/11. Every comedian in the world was treading on thin ground at the time--the death of so many people in such a horrifying way made comedy very difficult--even comedy that had nothing to do with any issue related to the catastrophe.
And Carlin has ALWAYS done social/political stuff.
What was he to do?
He confronts the issue right away, saying that he has to 'get something out of the way so we can have fun', and then goes into a comedic bit about how he would deal with bin Laden and his ilk. Unfortunately he literally can't be clever about it--the audience didn't want to hear what Carlin wanted to say, and he knew it.
So he made fart jokes instead.
Admittedly, when he gets going on his rhythmic lists of 'not just any guys, but...', he is very funny...but...fart jokes? It's not worthy of Carlin. That's a joke for 5th graders.
And I felt that the rest of the show suffered the same problems, and had the same strengths. When Carlin gets fired up on his variations on a theme (the "and here's another guy who should be strangled at birth with his own umbilical cord..." sort of thing), he's both outrageous and hilarious. But he lost the audience several times--and at one point he drops a bit before he's finished, saying "I sense I've lost you."
I did not care for his 'why you shouldn't stop after you've hit someone with your car' bit, although it had some good moments; and the end of the show was very weak. Now, this audience loved Carlin, they came knowing him and they were fairly positive even on some of his weakest material--you could tell they wanted to laugh. But there were a surprisingly large number of times when they just didn't.
Having said that, I must also give Carlin credit for having been one of the most polished comedians around--he knew his material backwards and forwards and upside down on fire, and there was never a stammer on the rhythmic delivery of some very lengthy and complex material. Any comedian would do well to study this performance to see how that is done by the master--ever since his "Seven Words You Can't Say on Television" bit, this has been Carlin's great strength.