Gears support in WordPress.com means you will be able to store all images and other web page components of the WordPress administration panel locally which will speed up access to those files while reducing unnecessary web traffic. To enable this feature, click on the TURBO link in the WP.COM dashboard area. This will be located in the upper right portion of the screen. You will need to have the GEARS extension installed before you will be able to take advantage of this new feature.
[EDIT] Rundown on Fool’s Wisdom of Google Gears Privacy concerned raised. Thanks Lloyd!