The devices will be exactly the same - well, they might come in a different box, but that's about it.
Frankly, the "launch" is more about marketing and PR than it is a technical event - more of a debutante ball than a birth typical of more conventional products. If you're waiting because you think the devices will somehow be different, it would only be that they'd come from the factory with more "final" software than the current chumbys - which will all be updated over the air anyway.
Current chumby owners are actually influencing the development of the product, as evidenced by the features in the Control Panel beta releases.