Progress indicators that moving perfectly and steadily are probably time based, but I'd have thought they've always been work based. Example: you know you have to copy 20GB across the network; update the progress bar with 'percent of data complete' periodically (on a timer, in the byte copying loop, whatever.)

I've seen dual progress bars: one for "overall progress" (each one of five tasks fills this bar 20%) and one for the current task (maybe downloading, maybe unpacking, maybe copying, maybe waiting for hardware to reply...)