A friend of mine - let's call him "Reign of F-ing Genius" - sent me an email weighing in on my post yesterday regarding using characters in the public domain:
Sadly, I am here to report that public domain is not nearly so clear cut when it comes to characters that have been adapted into film. The specific example I'm thinking of is the Invisible Man. H.G. Wells has been dead long enough that the character is technically in the public domain. Alan Moore put him to excellent use as a member of the League of Extraordinary Gentlemen. However, Universal still holds exclusive film rights for the property. So, when Fox adapted League into film, they legally could not use the same character from the book. So, in the film version, he's not THE Invisible Man, he's just AN invisible man. He's got a different name and the movie avoids specific references to his past. In this case, the change made as much difference to the final product as a leper catching a bad case of sniffles, but it goes to show that once a corporate entity has gotten its mitts on an existing character, the ownership issue becomes significantly more complicated.
Just something to keep in mind. I admit I'm not a lawyer, so do your homework when playing with characters of legally dubious status.