I recently finished a JSF 2 application + Twitter Bootstrap (http://twitter.github.com/bootstrap/) + Font Awesome (http://fortawesome.github.com/Font-Awesome/).
I really like the way final application looks and now I feel that ExtJS applications's UI looks too "dense" and too 90s... (colors, frames, etc.) so I start to use default gray theme and start to tweak, via CSS, the L&F.
One of the thing I liked the most in the JSF application was the icons provided with Font Awesome so I decided to try them in SA. Was really easy!
1. Create a new project and save it.
2. Add "fonts" and "css" folders to where you saved your SA application.
3. Copy "fonts" folder content from Font Awesome distribution to SA application's "font" folder.
4. Copy "css" folder content from Font Awesome distribution to SA application's "css" folder.
5. Add a resource to your app pointing to "./css/font-awesome.css"
6. Use the icon-* classes, for example:
Works in 2.1 and 2.2 beta.
Hope this be useful for someone.