I took my macbook Air to genius bar to get the hard drive sound checked out. I definitely hear it when in quieter environment.
Funny thing... Almost impossible to hear it at the loud Apple store. The "genius" took it to the back and said it was normal/acceptable HD sound.
My wife thinks I am crazy, trying to find fault. I guess I want perfection. I definitely hear it frequently, even now while I type this.
BTW, after 10.5.2 update, keyboard backlighting is more predictable, the fan is now rarely above 3500 RPM, mostly at 2500 RPM when doing normal activity. Even JAVA windows open does not send the fan up to crazy level. The genius when he was "stressing" my Air opened every friggin application (seemed like 20+) on my Air and surprisingly, the fan stayed below 3500 RPM. Go figure.