I'm wading into this w/ a little trepidation, but here goes...
Before my current PowerBook, I had a TiBook - a Titanium PowerBook, for those too young or too new to Apple to know the "lingo." With that computer, there was a somewhat similiar problem, in that the hand rest areas, which were really painted carbon fiber and NOT titanium, had a tendancy to "bubble" and peel, revealing black underneath. This was well documented on these forums (although most of those posts have expired). I bring it up because it may serve as a cautionary tale...
It took a lot of effort, by a number of people, to get Apple to recognize and finally agree to correct the problem. Many of the same types of comments you see here were made, like "You must have toxic sweat, from eating/drinking/smoking X" or "it is your fault for doing X/not doing X/wearing X" etc. Some of the Apple faithful (I consider myself one; nothing but since the Apple II+!) get irrationally aggressive whenever someone so much as complains about their computer. The company's initial stance was that it was a "wear and tear" issue. Through persistance and documentation, users were able to change their point of view on this, and the company did eventually agree to fix the paint on computers under Apple Care.
So why do I bring this up? I think a few lessons learned then can apply to the current situation...
#1 - Apple will come around, but only if you keep the pressure on. W/ the TiBook paint issues, threats regarding lawsuits were made - I don't know if that made the difference or not. Relative to that problem, the MacBook discoloration one seems to develop more quickly, so more people should be able to document it. Moracity is right - you need to call Apple, bring your computer in, etc. BTW, some Apple employees will say, "This is the first I've heard of this," long after this could be true. Be polite, but be persistant.
#2 - MacAddicts will criticize you. Maybe it is from all the years of people saying, "Why in the world would you use a Mac?" but Mac users tend to be a fiercely loyal, even overprotective bunch. Posters will suggest it is your fault, or that it is all in your head. It isn't, of course. But trust me, the best thing to do is to just ignore them; don't waste time, as I did, comparing who has the bigger "Mac cred" and getting drawn into a flame war.
#3 - You have the right to have a computer that doesn't start to have problems right away. Sure it is a cosmetic problem, and the computer probably works just fine. But who wants to stare at pink/orange/gray computer all the time, especially if you just bought it? Furthermore, as an Apple consumer, you become a (potential) Apple spokesperson. If you open up your computer and people all say, "Gross!" that hurts Apple, too.
In short, once enough people complain, loudly but coherently, Apple should come around. It ***** having to mail in your computer for a fix, but you can always keep the stains if you don't want to part w/ the computer and they don't bother you. Hope this helps.
PowerBook G4 Mac OS X (10.3.9)
PowerBook G4 Mac OS X (10.3.9)