Well, i just got my Mac Mini, and it is practically silent. Honestly. Even when transfering 15 GB of files like I did last night, I didn't hear the hard drive at all, to the point that I thought it might have gone to sleep. The only time it makes noise is when it's reading a CD or DVD at first.
My dad has a G5 PowerMac, though, and it's a little bit louder, but not by much. It's one of the first 1.6Ghz SP's, and it runs really quiet too. Much quieter than our old G4's and G3's, and definitely quieter than my G4 PowerBook which blasts its fan every five minutes.