That said, this comic doesn't present the joke in a vacuum. Randall needs, of course, a context for the idea, and he went with 9/11. Let's talk about that.
First off, I'm not opposed to making fun of 9/11. I think it's important to laugh in the face of these sorts of things, and that, especially in the case of terrorism, the way to not let them win is to not be scared, and laughter and jokes help with that. That said, it's still 9/11, 3000 people still died, 9 years ago, and the direct ripple effects of that are still with us. So what I'm saying is, at least for the next many years, make fun of 9/11 if you wish, but be careful about it, and make sure the joke is worth it. I know I've linked to this before, but here is the online version of the Onion's immediately post-9/11 issue (two weeks after 9/11). You should read all the articles. They are done brilliantly, combining humor with sympathy in a way that quite frankly I think every comedian needs to study, and hard.
Let's compare it to Randall's jokes about 9/11. A cursory search of my memory and xkcd's shitty, shitty search engine* gives three: a throwaway reference, a throwaway alt-text reference, and the two more interesting ones, a punchline and a joke in a long series of jokes. It's those last two that are more interesting. Comic 102 always struck me as really jarring - I don't usually expect such dark humor in xkcd, or most places. Think about it: the joke is her dad died on 9/11. Funny in some ways, but you really have to be in the right mindset.
(update: xkcd's crappy search function made me miss another one: 647. I'd call that one pretty much neutral, using 9/11 as a historical benchmark in the way many sudden tragic events, like the Kennedy assassination, are used).
(later update: Ian Jones points out another 9/11 comic, even a 9/11 truther comic, 258. It's not funny at all, but the point is, these jokes were funnier back then, in the first half of 2007. Now, they are a little old. Hell, even the birther conspiracy folks are old. What's a good new one?)
* Yes, I know Randall's search engine is Oh No Robot, made by man-god Ryan North. But he needs to freaking update it and also not constantly repeat comics in it.
And Secretary Part 3, with its joke about the 9/11 Truthers, making it a good one to compare to today's comic. I like the older one more - claiming 9/11 was a hoax because it didn't happen is a pretty absurd idea, and I like it - it parallels the absurdity of the 9/11 Truthers. But this one just isn't as interesting.
Today's comic is of course new ground in xkcd-9-11 jokes, because it has an actual drawing of the twin towers. Like the subject of 9/11 itself, you have to be careful. Here - let's remember what he is drawing:
I really really don't want you to think I'm opposed to making any sort of joke about a tragedy - that's why I stressed so hard that it can be ok. But seriously, make sure it is worth it! Was this comic worth it?
I'm worried that 9/11 is quickly becoming the new all-purpose topic for jokes when you want to prove you aren't scared of making jokes about a touchy subject, a la the holocaust / hitler or dead baby jokes. Or rape jokes. Basically anything Cyanide and Happiness makes jokes about regularly. There are good jokes about Hitler - just watch The Producers. But they are vastly outnumbered by shitty jokes that people made because they aren't funny enough to think of good ones, and are substituting shock value for humor value. The fact that awkwardness or tension sometimes manifests itself as laughter can be enough to convince these people that they should continue to make such jokes.
Maybe I don't even dislike this comic - I'm not sure. My thoughts about its subject matter are getting in the way, but maybe that itself is proof that it's a bad comic. I'm also a little bothered that more people aren't concentrating on this part of it - they're just taking the context of 9/11 in stride and talking about the truth-lies-halfway-between-two-claims part.
I wasn't expecting this post to turn so philosophical (hey guys let's debate the justifiable limits of free speech in the comments!) so take it as a set of thoughts inspired by the comic.
ha ha, truthers.