The event is over, and Apple has just released the iPhone OS 4.0 beta firmware for developers and this OS is available for iPhone 3G, iPhone 3GS, iPod Touch 2G and iPod Touch 3G. OS 4.0 will be released for the iPad this Fall.
The latest iPhone OS 4.0 Beta 1 is build 8A230m. Remember, if you want a jailbroken device, then this update is not for you. You will have to wait for the dev team to release an OS 4.0 jailbreak.