A thread was started earlier today, titled "What is your favorite photography joke?" There was some discussion going on in the comments of this question about whether the question was off-topic, and if it was off topic, was it an acceptable diversion from the primary purpose of the site.
As a user of StackOverflow for over a year, and a user of SuperUser and ServerFault as well, I think that some threads, like "favorite jokes", "favorite quotes", etc. are the kinds of topics that fall on the edge.
Personally, I enjoy joke and quote topics. I think they add a needed fun element to these sites that help keep the mood lite. However, where should the line be drawn, and when do such topics become truly off-topic, and worthy of closure or being moved to a more appropriate site?