From a cursory reading, this isn't particularly new, even on the web. Essentially what he's done is implement bindings/key value observing, but with a new syntax instead of the one we already know (i.e., javascript). On the desktop Cocoa has done this very well, and on the web Cappuccino and SproutCore have copied the cocoa approach. There are also more lightweight libraries that provide binding support without a heavy framework.
The difference is the realtime synchronization across all clients. Fun comes with a complete network stack, ensuring that the state you're viewing is always up to date. Does that make sense? I actually do think that's a new value proposition.
Still, creating a new language just for bindings (across all clients) still doesn't make sense to me. A good javascript library or framework should be good enough.
