Well, what more can I say - Gmail is now powered by Gears. Read about the announcement and details here...
Google started off with reader and now gmail. I see them enabling all their apps with Gears and this is the next step in making all the desktop apps irrelevant. And they have all the right tools and pieces in hand to achieve that. The tools - Gears, the client - Chrome (& firefox?), the apps - Gmail, reader, ...
Would I be afraid if I were MSN, Yahoo, Facebook, AOL... or any other web app out there? Yes, certainly I would. And I personally would hate if Google goes ahead and dominates the web. Offline availability is so important for every web app out there. Atleast in the short run, all the web apps should try to enable offline access. Myspace saw the need for offline access to information and did the right thing by enabling their message/mail access offline.
Now I foresee many more web apps enabling offline access via Gears, or otherwise. HTML5 is still long time away before being ubiquitous. And Gears will fill in, till such a thing happens and continue to provide the new features coming in HTML5, right NOW.