My Cellphone is as powerful as a 2002 state-of-the-art desktop machine. There are problems here, but this is not it.

Oh?

Resuming downloads, large downloads (can't decrypt and stream to the disk as it comes in), and handling any future changes to the encryption algorithm (say, asymmetric instead of symmetric)? How about that, as it's in JS / attached to the DOM, an injected script has access to it? Or the several gigabytes of memory it would take to store and decrypt a gigabyte download? It'd also likely end up being an even bigger battery drain than Flash.

It's a definite problem. It won't be for long, I think, but it most certainly is now.

Network latency and bandwidth is the new MHz.

Actually, I think that may have been the one after MHz. Now we're into how many cores something has.

Downloading client-side generated files is possible using data-uri. However these are usually small; it would be very difficult to store a 1GB file in memory (in javascript) while you decrypted it and I doubt data-uris that large work across different browsers.

True, but there's no way to specify the filename and extension, so in practice you have to use Flash.

It depends on the browser. Safari 5 works, but Chrome doesn't, not sure about Firefox, and I suspect IE doesn't.

I use it on my torrent-conversion bookmarklet on http://hid.im.

Not quite true: http://jszip.stuartk.co.uk/