Well, as excited as I am about SproutCore (it does look cool), coming from a newbie-cocoa programmer who hasn't done much Javascripting and hates touching HTML & CSS, I don't think SproutCore is the way to go here; it might of been 2 weeks ago.
That was before objective-j. You've probably seen the news on it; 3 college grads, two of whom recently did some work for Apple, wrote an entire implementation of the Cocoa-frameworks, or something very very very similar thereof, in javascript, and are vowing it'll be released OpenSource very soon. Apparently they just want to tie up some loose ends. The reason why "Cappuccino" (getit? A mix of Java and Cocoa; cute, I know), their Objective-J framework, is cooler than SproutCore is it doesn't use any Ruby at all, and allows developers to build rich-web interfaces, full programs even, and deploy them without ever seeing a single piece of HTML. It builds just like a desktop app, and thanks to their implementation of the Cocoa frameworks, you can even code-share between your Cocoa desktop app and your Cappuccino web app!! Even better yet, since it's a strict implementation of Javascript, you can still have access to all the underlying language features, just as in Cocoa you can still use the base-level C language (if you want).
Because of the way it's built it too requires no new downloads for end-users; if their browser supports Javascript, it'll run no problems. Apparently it even works in IE6!!
I think we can expect big things from these guys. If you've not seen or heard of it, there's a tech demo app they built for making powerpoint/keynote presentations in a web-browser! This web-app is better than many desktop apps and runs in about the time it takes to launch powerpoint. You simply cannot get any better!!!
Here's their blog: http://blog.280north.com/
And here's their tech-demo (which apparently isn't even finished yet!):
http://280slides.com/