All of us who complained to Apple about this health hazard should be pleased to see the fruits of our labor. I had actually reached the point of contacting lawyers and inquiring about a class-action lawsuit because I thought Apple would do nothing. I'm happy to be proven wrong!
Several people here have commented that Apple never promised quiet machines. This is incorrect. Apple has stated unambiguously in their own policies that they consider environmental noise to be hazardous (in agreement with all current scientific research on the subject, not to mention ISO standards) and that they intend to build quiet machines. While there will always be some subjectivity involved in what one considers "too loud" or "quiet", when noise levels exceed the threshold of physical pain for a substantial percentage of users, as do the original MDD's, I think it's quite reasonable to claim that such a product is "too loud." In the future, we will undoubtedly have a standardized method of rating noise emissions (just as we rate CRT radiation emissions now) and legal limits on how much noise is allowed for specific intended uses and environments. Apple has always been on the leading edge of this kind of effort by self-regulating the noise emissions from their machines and, again, have stated as much in their own policies. So, when a company explicitly claims that they produce quiet machines, it is reasonable to expect them to live up to those claims. BTW, I bought my MDD 867 from an Apple store and it seemed dead quiet there. I hadn't read about any noise problems and, once again, due to Apple's own claims and history of quiet machines, didn't think to bring an SPL meter or anything. stupid me
g4noise.com was/is in the process of proving that the MDD's are a health hazard because they cause physical pain (headaches). So hooray for them. I think Apple saw the writing on the wall and knew that they might lose an impending product liability lawsuit (or at least didn't want the bad publicity). Conspicuously, Apple never published official noise specifications for the original MDD's, probably because they violate the ISO specs. Also, the fact that Apple bragged about making the latest generation of MDD's considerably quieter is tantamount to admitting that there was a design flaw in the original MDD's. These are just a couple of indicators that show Apple's potential vulnerability to a lawsuit.
Anyway, I think this "fix" offer from Apple is absolutely fair (and what most of us were asking for from the beginning) The $20 fee is probably called "shipping and handling" for legal reasons - you can't ask for a much better legal redress than a "free" fix, right? - but, again, I think it's fair. In fact, I give Apple credit for keeping this price so low as it must represent a slight overall loss for them (or break-even at best). However, I give us "compainers" the most credit. If not for our vocal outrage, we never would have seen this offer from Apple.
I am utterly confused by some of the posts here by people who would have us suffer physical pain in deference to some kind of bizarre allegiance to Apple. I love Apple products, but I don't live in a fantasy-land where I pretend that Apple is all-knowing and never makes a mistake. I will not tolerate a product that negatively affects my health. Anyone who would tolerate such a thing is uninformed of or simply doesn't care about their rights as a consumer. I feel sorry people who fall into that category but I don't condemn them and I can't understand why they would want to silence those of us who reasonably demand satisfaction for a real and quantifiable product malfunction. Apple thinks the noise problem is a real malfunction, otherwise they wouldn't have fixed it in the new MDD's. Ultimately, Apple also decided that our complaints were justified, otherwise they wouldn't have offered this replacement at such a low price. So who exactly are you angry with? Apple, for agreeing with us? More importantly, why do you care?
So, power to the people

- we win and, in the long run, Apple wins. yay!