There are already some implementations done – offline Google Reader will be available shortly and Dion Almaer has already geared RSS Bling offline. Dion also notes that Dojo Offline has already started using Gears as a platform.
This can bring Microsoft in a challenging position, since this makes many Web applications more accessible now, especially the online office suites.
One thing to note would be performance. Firefox is already burdened with many many addons, will this make the browser usable?
Gear For Offline | iface thoughts