Or manipulating user perceptions for fun and proffit

All programmers know that if you have a blocking task that takes N seconds, the user will complain that it takes forever. So if you add a progress indicator that ends up making the task take N*1.25 seconds - they will compliment you on how faster it went.
Remember the phrase "Time flies when you're having fun?" The inverse is also true. Time slows when your not having fun. Ever been stuck in traffic?

Here's my variation on the theme: Unless >50% of your product's users are uber geeks that need you to be counting bytes with hyper accuracy - i.e. downloading, reformatting drives, defragging, ect. AND it's a task that generally takes less than a minute, why not max out your progress at 90%?

It seems to me that most users generally don't care about the mundane.
Do they care if it gets to 100%? No.
Do they raise an eyebrow if some marketing bozo / football coach doesn't understand that you can only give 100%? Anything more would be subtracted from nothing? (This is a personal pet peeve of mine BTW) No.
Do they get annoyed when it "hangs" at 100% or goes beyond 100%? (i've seen it in the wild) yes

All they really care about is that the task is that there has a pretty indicator telling them that it's working and that it's now done. So why not give them what they want and they'll return the favor by raving about the "responsiveness" of your product ;-)

Yes, I know it's deceptive - but I can deal with that if users are happy.

Oh, 90% is over the edge? then how about we all promise to never go over 99% unless we are truely 100% done with the task and we are not blocking user interaction? I known that I hate it when an installer gets to 100% and "hangs" for reasons unknown - don't you ;-)