I've been out of the OSX-space for a while. Are there a lot of apps using web views for their GUI? The only ones I can think of are Colloquy and Adium. Even then, they don't do the whole GUI as a web view, just the parts of the display that are theme-able.
Maybe I should have specified WebView rather than web view. IIRC, a WebView is a ready-to-go embed-able WebKit widget in the toolkit of OSX GUI developers.
Colloquy represents the chatroom text as a stream of XML, which it uses XSLT themes to translate into HTML, which is displayed in a WebView. I'm sure Adium does something similar.
"Using HTML/CSS/JS as the new GUI medium" doesn't limit itself to just running stuff in a traditional browser. I was under the assumption that we were talking about making HTML/CSS/JS the new GUI layer, even for native apps.