Google Gears has been enabled on WordPress.com for a couple of weeks now for some members, but was only announced this week. Andrew Ozz (azaozz) added this feature a couple of month ago in the development version of self-hosted WordPress. I’ve been using it for about a month, and even though I have a decent internet connection (15156 kbps measured), I really notice how quick Gears makes the visual editor’s Insert Link popup pop. Over all, it feels a little quicker.
Reading some of the comments there is some confusion about whether this allows an offline mode of WordPress and also about the privacy of using this Google browser add-on.