Yup, seems like this is rather competing with the likes of GWT, qooxdoo, ext and dojo.
I'm not crazy about the OSX look either (is it skinnable?) but I liked the responsiveness of the demo apps. Also generally all entries are welcome to this space - it's anything but a solved problem.
Yes, some of the recent stuff we've been working on will making skinning much easier.
We now have two themes, the "default", which you see in 280 Slides, and the new "Aristo" theme we announced with Atlas (designed by Sofa: http://www.madebysofa.com/)
Aristo is designed to "feel at home" in Mac OS X and Windows. Does it look too much like OS X for you, or are you talking about the 280 Slides theme?
HTML/CSS/JS is always skinnable, isn't it? Failing all else, you just throw in a few jQuery calls to skin your finished, Atlas-designed Cappuccino app... :)
I'm not crazy about the OSX look either (is it skinnable?) but I liked the responsiveness of the demo apps. Also generally all entries are welcome to this space - it's anything but a solved problem.