The Simpsons, in addition to being a long-running source of comedic genius, has also proven to be a veritable gold mine of copyright anecdotes. The DVD commentary tracks are filled with stories about rights clearances (a running theme of which is a decided preference for using FOX-owned properties, since the rights are easier to clear).
Well, the commentary to A Streetcar Named Marge (from the Fourth Season has a great one. The reason that Marge appears in a musical production of "A Streetcar Named Desire" is that they couldn't clear the rights to use the play itself.
Instead of scrapping the episode, the writers sat down their lawyer and asked what they could do without the permission of the copyright holders. Well, under fair use, they definitely could quote a line or two. And writing songs that referred to the story, without actually copying it, would be okay, too. Indeed, the lawyer said he'd be happy to take the case up to the Supreme Court if sued.
The rest we know. The writers cranked out "Streetcar!" (working name "Hello, Trolley"), and though they offended New Orleans mightily, the episode was a hit, and parodies in the form of musical remakes became a Simpsons standby, from Shari Bobbins to Stop the Planet of the Apes -- I Want to Get Off.
There's no simple moral here for copyright partisans. The writers certainly attribute their creativity on the episode to having to work around the roadblock that the refusal of permission created. At the same time, the show's genius depends critically on fair use. Perhaps this, then: both reuse and transformation are important parts of a healthy artistic breakfast.