My mini sits on my desktop beside a 24" display. With 2.26GHz processor, 7K320GB Hitachi HDD and 4GB ram, the mini purrs along with the fan at 1495 rpm (read in iStatPro widget from iSlayer.com), which, in my environment, isn't noticible. So far, even while editing huge Photoshop files, I haven't heard my mini. On the other hand, my 2.2GHz MacBook (also with 4GB ram and 7K320GB Hitachi internal) frequently spins it's fan up to a very noticible level when I do some more intensive work and I really don't like that about the MacBook, but I tolerate it because it's such a great little laptop!
Unless you are in a completely quiet environment, most people would consider this a very very quiet computer. Mine has the 500GB HD and 4GB of ram and it is very quiet even after 2 + hours of file copies and fans at 1750 rpm. (of course it's not really a quiet environment here so the ambient noise may mask the fan noise).
Cheers,