[DougWebb]

That's not a progress bar though; the visual metaphor of a progress bar is a container/tank that goes from 0% 'full' to 100% 'full'.

What you described is a more general status indicator, bytes received, which I would display along with a transfer rate. That should not be shown as part of a progress bar because you don't know the percentage. However, I do agree that it's very useful information, and I always watch that kind of status indicator on FTP/SCP/torrent/copy+paste file transfers.

Unfortunately, browsers don't provide great feedback info to javascript even for that basic information about an ongoing request, if they provide any feedback at all. Usually you just get 'started' and then 'done'.