I’m a little surprised that I’ve been blogging for almost five years now and never got around to talking about whether there’s a difference between the words disinterested and uninterested. I suppose I’ve avoided it because the matter has already been excellently discussed by many others, and I didn’t think I needed to add my voice to that choir. But now it’s become something of a glaring omission in my mind, so it’s time to fix that.
Let’s skip to the end and fill in the middle later: there is a difference, but in Mark Liberman’s words, it’s “emergent and incomplete, rather than traditional and under siege”. For some people, there’s a clean separation, for others an overlap. In the language in general, uninterested is limited to the “unconcerned” meaning, while disinterested can mean either “unconcerned” or “unbiased”.
How do two distinct meanings arise from such similar words? The problem lies at the root — namely, interest, which can be with (1a) or without (1b) bias:
(1a) I espouse a relatively dull orthodox Christianity and my interest in Buddhism is strictly cultural, aesthetic.
(1b) Upon consignment of your car, it’s in my interest to do everything possible to present your car to potential buyers.
So, when one adds a negative prefix to interest(ed), is it merely disavowing concern, or bias as well? I don’t know of any inherent difference between dis- and un- that would solve that question, and historically, no one else seemed to either. Though I don’t have relative usage statistics, the Oxford English Dictionary cites both forms with both meanings early in their history:
(2a) How dis-interested are they of all Worldly matters, since they fling their Wealth and Riches into the Sea. [c1677-1684]
(2b) The soul‥sits now as the most disinterested Arbiter, and impartial judge of her own works, that she can be. 
(2c) He is no cold, uninterested, and uninteresting advocate for the cause he espouses. 
(2d) What think you of uninterested Men, who value the Publick Good beyond their own private Interest? 
But we both know that it’s no longer the 18th century, and I strongly suspect that you find (2d) to be a bit odd. The OED agrees, and marks this meaning (uninterested as “unbiased”) as obsolete. I looked over the first 50 examples of uninterested in COCA (Corpus of Contemporary American English) as well and found no examples like (2d). If it still exists, it’s rare or dialectal. Uninterested meaning “unconcerned” (2c) is, of course, alive and well.
So really, it’s not a question of whether people are confusing uninterested and disinterested, but rather a question of whether disinterested has two possible meanings. We’re certainly told that they are, and that it is imperative that disinterested be kept separate. For instance:
“The constant misuse of disinterested for uninterested is breaking down a very useful distinction of meaning.”
Is it really? Suppose disinterested could just as easily take either meaning, and that this somehow rendered it unusable.* You’d still be able to use unbiased, impartial, objective, or unprejudiced for the one meaning, and indifferent, unconcerned, and uninterested for the other. We’re not losing this distinction at all.
Setting aside such misguided passion, let’s look at how disinterested actually is (and has been) used. As we saw in (2a) & (2b), disinterested started out being used for both meanings. This persisted, according to the Merriam-Webster Dictionary of English Usage (MWDEU), through the 19th century without complaint. Noah Webster’s 1828 American Dictionary disinterestedly lists both senses, and it’s not until 1889 that MWDEU finds the first complaint. Opposition to disinterested for “unconcerned” appears to have steadily grown since then, especially in America.
But despite all the grousing, “unbiased” disinterested is hardly in dire straits. MWDEU’s searches found that 70% of all uses of disinterested in their files between 1934 and the 1980s were of this sense, and that this percentage actually increased during the 1980s. Furthermore, the MWDEU notes that the use of disinterested for “unconcerned” usually has a subtle difference from uninterested. Disinterested is often used to indicate that someone has lost interest as opposed to having been uninterested from the start.** This fits with other un-/dis- pairs, such as unarmed/disarmed.
Summary: Far from losing an existing distinction, it seems that we’re witnessing a distinction emerging. Uninterested is now restricted to an “unconcerned” meaning. Disinterested covers impartiality, but it also can take the “uninterested” meaning, often indicating specifically that interest has been lost. Because many people object to this sense of disinterested, you may want to avoid it if you’re uninterested in a fight. Will the distinction ever fully emerge, and the overlap be lost? Would that this desk were a time desk…
*: I think it goes without saying that having multiple meanings does not make a word unusable. In case it doesn’t, consider the much more confusing words fly, lead, and read.
**: Compare, for instance, I grew disinterested to I grew uninterested. I definitely prefer the former.
**: MWDEU notes that while the distributions of the two senses overlap, it’s more clear than people let on; “unbiased” disinterested tends to modify an abstract noun like love, whereas “unconcerned” disinterested tends to modify humans, and appear with in in tow.