If there is a third speaker in the MacBook Pros just as the previous Powerbooks (I've heard reports stating both 2 and 3 speakers are present), then Apple's implementation of the sound balance is incorrect. When a single bass speaker is available, it should always be at the same volume regardless of the sound balance selected. This way, whether the sound is balanced to one side or the other, it would still have the same frequency response and thus the same tonal characteristics. Since the tone changes with the balance, either the left and right speakers have different frequency responses (if there are only 2 speakers), or a third bass speaker isn't acting as it should.

Again, this is never really noticable in my case, because I always keep the balance set to the middle and hardly ever voice/video conference over iChat, but I still feel that Apple should resolve this issue.