Already the GMail mobile web app works offline with both Gears (Android 1.6 and earlier) and HTML5 (iPhone Safari, Android 2.0). The non-mobile Google Reader site has an offline mode for that works with Gears in Chrome, Firefox, Safari, and IE.
The Chrome OS webcast made repeated references to data cached locally. I'm not sure but it sounded like it might even cover more than just apps specifically written for Gears/HTML5.
