Followup:
Before posting my original message, I did compare the sound of the spacebar against two plastic MacBooks, a Dell laptop, and an Apple aluminum "chick-let" keyboard. My new aluminum MacBook's spacebar was the loudest of all of them.
Last night I went to the local Apple store and tested several of the laptops there. I tested several new Aluminum MacBooks, a MacBook Air, and a (late 2008) Aluminum MacBook Pro. There was quite a bit of ambient noise in the store, so it was difficult to compare the loudness of the spacebar against what I heard in my quiet office at home. I did determine that the spacebar noise varied between all the notebooks I tried. I would say that a few of them seemed much quieter than mine, possibly approaching the level of my old plastic MacBook. However, several of them seemed potentially just as loud as my new MacBook (again allowing for the difficulty of comparing sound levels in a noisy store).
I spoke to a genius who told me that I could bring my MacBook in for a keyboard swap if I wanted to. He said they could do it in the store. I custom ordered mine from Apple with 4GB of RAM, so I would have to work with the online store if I wanted to replace the entire notebook. If Apple determined it wasn't a defect, I'd be charged a 10% restocking fee for the swap.
Because so many of the models in the store appear to have the same noisy spacebar, and with all the complaints about slanted function keys (which mine seems to have a few of as well), I've decided to just sit tight for now. In a few months when Apple works out the QC issues on the new keyboards, I may take mine in for a keyboard replacement. Until then, I'll enjoy using this new notebook. Even with this one issue, this is the the most satisfying laptop I've ever owned, and I'm very happy with it.
Thanks to everyone for all the feedback.
