It comes as news to most of my students, semester after semester, that disinterested can mean impartial — not having an interest, or stake, in the issue, not having a dog in the fight. They read disinterested as not caring, not concerned, not interested in the sense of having curiosity or concern.
If I were Professor (adjunct) Harumph, I could sneer at their subliteracy and parade my own vast erudition. But my job is to fit them for working effectively as writers and editors, so I explain the realities:
Basics first. Both senses of the word are in use. They are active in the language. The question is not whether one is right and the other wrong, but which is appropriate.
Levels of usage must be considered. In the conversational or colloquial level, disinterested in the sense of uninterested appears to be common, perhaps dominant. As the language of journalism has become more conversational, disinterested in this sense also appears more frequently in writing. In more formal, and perhaps I can risk saying more sophisticated, levels of writing, the sense of impartiality can often be found: academic writing, for example, or serious books.
And there is the social dimension to consider. There are people, members of a small but vocal minority, who attach class or even ethical values to word usage. These are the people who will think less of you for ignorantly using disinterested for uninterested. They are uninformed but dogmatic, and they are out there. There’s no need to pull the coverlet over your head and hide from them, but you shouldn’t be surprised if you hear from them.
I say uninformed. To become better informed, have a look at Mark Liberman’s thoughtful post at Language Log. As it turns out, disinterested/uninterested is not a case of a pristine sense corrupted by ignorance, but a much more common phenomenon in language: an incomplete differentiation emerging from a tangle of historic usages. This will frustrate the prescriptivists and copy editors who want to insist that there is always a Right and a Wrong in usage, but I can’t help them.
I can try to help my students by advising to keep in mind the basics of rhetoric: What are you trying to say? Who is in your audience? What words, tropes and strategies will convey your meaning to the reader most precisely, without misunderstanding or distraction? There are choices to be made, and editing is making choices.