Some jokes are as old as Methuselah. Actually, the oldest joke book is from the 4th century. It's a little text called Philogelos, and yes, we actually own a copy here at OSU.
You can imagine, then, that joke theft is also an old practice, and where there is theft, there is litigation. Where there is litigation, there is of course a law faculty member or two interested in studying the matter. Check out the newsy version of the faculty analysis, or read the Virginia Law Review article on the subject. As a brief note on the article, the faculty authors distinguish between joke theft and copyright infringement, which should be something warranting a "hmm..." from our blog readers who are also copyright enthusiasts.