Question for people using these progress bars: Are you actually doing the computation to accurately estimate the time it takes to complete the action indicated by the progress bar, or are you simply showing a progress bar as a visual cue to any loading in the same way people have been using spinning AJAX loaders?
compu-what-tion? You mean accurately predict how long it'll take an ajax request to load? Pfft, an exercise in futility, easier to just fake it, and probably just as accurate :)
A harder approach would be to be super accurate with actual size of response. Content-length on the header won't work because it only refers to that one file. Also, the server won't have any clue which files belong to which web page response. A way around this is to check every request against a session ID. You'll also have to pass back some sort of "request hash" -- GUID for the request so you can match different files from the same request (static files, content, etc).