A couple of people pointed out Macromedia Central to me as an alternative platform for developing rich apps. I spent some time just reading docs up on the website and I tried the demo. Looks quite good. Very straightforward API with good support for web services and a stateful UI. Check out the reference poster and architecture description. It even has the concept of program manifests. (Hmm, where have I heard that term before? :-)
ActionScript 2.0 looks like a variant of JavaScript that's adopted more Java constructs. It would be pretty comfortable to work with.
The cross-application integration seems rudimentary and you don't seem to be able to exploit any local services on Windows. However, all in all, it does look like an easy way to create a rich cross-platform client - at least simple informational apps but I'm not so sure about big enterprise style applications.
What I don't like about systems like this is getting trapped by limits of the language and platform. Of course, you're stuck if you want to be cross-platform but often you don't need to be in which case you really want a backdoor to local services. It would be great if this platform provided a bridge to .Net. How about F#?