The new Tweetdeck for the Mac is supposedly HTML5-based:
http://www.cultofmac.com/134237/tweetdeck-releases-html5-based-app-in-the-mac-app-store/
Anyone know how
My guess is they do something similar to what PhoneGap does - add a webview in an XCode project. Webviews are basically embedded browsers that can run html5 and javascript. Then, they code their pages in html5 and can call their dataservices via ajax.
Here is actually a tutorial on how to add your html5 code to a webview in xcode: http://www.lostdecadegames.com/how-to-embed-html5-into-a-native-mac-osx-app/