Apple wont help you if you ended up taking it back.
Just like when I took my MBP back for a squeaky spacebar. The Genius acknowledged the sound, but he said it was normal because the mechanism is metal on metal so it's not going to be silent...in other words, it's totally normal to have a squeaking spacebar for a computer that's 4 weeks old (C2D ordered a day after it was released).
Funny thing is, when he pried up the spacebar and showed me the mechanism just so the visit wouldn't be a total waste and after taking 20 minutes (literally) to put it back on the squeak is gone. So those of you with a squeaky spacebar, just take it off and re-seat it. Be warned: it is difficult to put back on even for a Genius.
The cause of the squeak? I think its the way they seat the silicone spring. The squeak did not sound like a metal/metal, rather a rubber squeak, like rubbing a balloon but at a lower level.
edit: oh one more thing, i mentioned macrumors.com and that other users had this problem. his response was that since it is not patrolled by apple techs, this site is unreliable. Sure enough, Apple.com also has squeaky spacebar threads, but i checked after i went back home in disgust.
Sorry for the digression, but basically Apple will say: Well, we can't put it on too tight otherwise you won't be able to press down the trackpad button. It's intentionally loose. OR Its a moving part, there's going to be some sound coming from it.