WordPress.com have added a Turbo button in the admin area. This is to enable Google Gears, if your browser supports it, and speed up things using it. For instance, Google Gears is used for offline access to web apps like Google Docs, but how can it benefit WordPress.com?
On WordPress.com it is used to store all images and other web page components from the admin area to the user’s PC, speeding up access and reducing unnecessary web traffic.
That’s from the launch post.
Upcoming WordPress 2.6, now in beta 2, will have the same kind of support for Google Gears.