I'm way more interested in using Flash only for communicating with XMPP and rather do the rest in HTML/CSS/JavaScript. Also with a fallback to BOSH, so it will work for those without Flash too.
This feels a lot like Juggernaut (Rails plugin) Although its not XMPP based. <a href="http://juggernaut.rubyforge.org/" rel="nofollow">http://juggernaut.rubyforge.org/</a>
It looks like the server-side stuff will be in Ruby (the About references Ruby inspirations), but I couldn't find anything specific on what you'd code other than XMPP and Flash.
I think this sounds awesome.<p>However it is odd they didn't link to GitHub or something. Looks like they just launched the site, so hopefully that's coming soon.