You can now download it from the Mac App Store!
Tumult Hype is the HTML5 creation app for OS X. The next major version is coming soon; join the mailing list or apply for the beta: http://tumult.com/hype-pro
The idea of separating the design from code has been swirling about for some time, and this will be another company's stab at the problem. Some other people trying to do the same thing are:
The tipping point is oh so very close.