As of right now, the official release date of the iPhone stands at June 15, 2007, though it could easily get pushed back much like the release of AppleTV. There are many factors that need to be in place for the iPhone launch to be successful, and Apple does not seem like a company who will release a product until it is absolutely, positively ready. This date (June 15) seems to be speculation from a document that Apple sent to the FCC:
A letter to the FCC from Apple dated October 12 requested the FFC to keep quiet on various diagrams, photos and user manual documents until June 15, 2007. This date fits exactly with the last day of the WWDC and could be the iPhone release date as well.
Letter to FCC from Apple regarding