I don't expect progress bar to accurately predict time - it's there to show progress. The obvious way of implementing it is to split the task into milestones of some sort and display how many are completed. It helps if there's enough milestones so that the bar looks smooth and each one takes roughly the same time, but it doesn't have to be.

Even for time estimates - I don't sit and watch the bar move from start to end. I'll look at it for a few seconds, notice how fast it moves, and go do something else. That "psychologically friendly" progress bar that starts slowly and accelerates will do nothing but confuse me.