Apple has not announced a specific release date. All they've said is "summer". You can believe the rumor posts such as the one JGG linked to if you want to, but they're all just speculating and have no inside knowledge. That particular rumor is actually basing their guess on poor information: the 2.0 version of the iPhone/iPod OS was released in July, so the fact that the 3.0 version was released in June means little if anything.
Very much agree with Dave Sawyer, Basing oneself in rumors is not often reliable enought to be consider proof. I do agree in something that given apple past history of software releases we could expect the Iphone OS 4.0 to be around somewhere between last days of June to the first days of July.
However at the WWDC '10 apple would give the public a date for the new iPhone OS 4.0 to be released for the masses to download to their portable devices. Along with an updated version of apple phone.