I'm a bit torn on this whole issue, myself. On the one hand, I agree that there should be more decent literature with minorities in non-stereotypical roles, but on the other hand as a writer I take issue with anyone telling me what to write (or what I ought to be writing about, as the case may be), and I don't like the idea that an author should be criticized for their choice of characters. Also anything coming close to a quota system will only encourage tokenism, which I think we can all agree is generally a bad thing ("Oops, better put a black character in here or I'll get panned", it's not exactly a healthy writing environment).
Though I agree fully when it comes to film adaptations changing a character's race, that's kinda scummy. I think characters should be cast as close to the description given in the source material as possible.
(Incidentally, the Otherland series and The Girl Who Saved the King of Sweden both feature black South African women as main/major characters and both happen to be incredible. Dunno if that counts as "minority" due to the setting, but still! Highly recommended, both.)