Preview at WWDC likely in June, followed by September launch.
Apple Posts New iOS-Based AppleTV Firmware
The firmware is of no use to anyone just yet, except perhaps those looking to jailbreak the Apple TV. But with the new Apple TV already shipping to customers, those individuals should be able to restore their Apple TVs to a factory fresh state using this firmware. Existing Apple TV owners are unable to take advantage of the new firmware due to fundamental differences in the hardware between the devices.
Given the shared codebase as iOS, many are expecting that the AppleTV will be jailbroken in time, opening the door to 3rd party applications and other hacks. Steve Jobs has said that they could open the door to an Apple TV App Store when the time is right.