The parody situation is quite interesting, in the US at least, in that you can do a parody of a material once. If you do another parody of the same material you can be taken to court over it.
I got to sit in a lecture of it once, using the example of Disney vs the Air Pirates. From what we were told of the case, if they'd left it at one issue, there wouldn't have been a case, but they released a second issue which is how they got nailed.
Brief overview of it (which doesn't quite match what we were told by the lawyer, but hey, its Wikipedia):http://en.wikipedia.org/wiki/Air_Pirates