My girlfriend also has the same problem as everyone else here is mentioning.
We went into an apple store to have it resolved as we have literally had the phone for 6 hours.
We had a pretty terrible experience with all of the apple representatives we had, between them telling us that they wont even look at the phone without an appointment, and that they "sold all the iPhone 4's" so they couldnt even replace ours if they wanted (obviously this was BS)
After 30 mins of poor service, complaining about not being able to even make a speakerphone call on a BRAND new phone, we finally got them to bring out a technician whom immediately duplicated the problem and was happy to get us a replacement phone (he was excellent).
I should also mention that one of the Apple associates jumped into one of my conversations with another associate just to mention that I was wrong and the mic on top was used for nothing but noise cancelation. I work in retail, and its one thing to provide someone a spotty (my experience was terrible) service experience. However, to call me a liar when you are pretty blatantly wrong is a very very poor use of judgement.
Long story short, up to this point, none of the apple associates had heard of this problem, and just may give you a fight. Don't take any crap and make sure they replace your phone. Good luck, hope everyone has their problems resolved!