I guess it's typical but I've had this idea before. tptacek should stop by with his rant against javascript, client-side encryption.
The other trick to making this idea cool? Once you have the wiring hooked up to make Mega look like local storage (even though it's in the browser), you can wire that up to the PeerConnection Data Channel (once it's available) and literally implement peer-to-peer networks in your browser. There are limitations as you can't use aggressive discovery protocols like with a native socket, but it's still tantalizing.
The other trick to making this idea cool? Once you have the wiring hooked up to make Mega look like local storage (even though it's in the browser), you can wire that up to the PeerConnection Data Channel (once it's available) and literally implement peer-to-peer networks in your browser. There are limitations as you can't use aggressive discovery protocols like with a native socket, but it's still tantalizing.