Not at all, Apple still holds the warranty. I exchanged my early AT&T preorder unit with a new one this weekend from the Apple Store. It took some doing and they tried getting out of it claiming no stock. Then, I told them I called ahead to make sure they had stock and gave them the name of the person that told me they did. I had to persuade a manager this as well and all of a sudden they come out with one claiming they just got more in. I suppose that could all be the truth but I just don't think they wanted to exchange it.
In the end, I too can report the new phone is performing much better than the old one. Night and day difference, I am completely satisfied with this phone now.