Here’s a somewhat fluffy NYT Magazine piece from Daniel Engber, titled Who Made That Progress Bar? He credits it to an interface designer named Bob Stahl. I found this tidbit interesting:
Myers asked 48 fellow students to run searches on a computer database, with and without a progress bar for guidance. Then he had them rate their experience. Eighty-six percent said they liked the bars. “People didn’t mind so much if it was inaccurate,” Myers says. “They still preferred the progress bar to not having anything at all.”
It lets the user know there’s magic happening behind the curtain.