AO did you get around to switching the HDD? Any other Mini users have similar problems with the noise levels? Noise level aside, the Mini is definitely a cool machine, and it's so small!
I did crack open my mini and put in a 1 gig patriot stick of pc3200 (150 bucks at newegg and works great), but I haven't swapped the hard drive yet. The whine is definitely the drive in there. I'm still planning on swapping it eventually, but both of the external enclosures I ordered turned out to be DOA, so making a disk image for reinstalling hasn't happened yet.
I'm also planning on using a 3.5" firewire enclosure to store all of my music, and am thinking about using it as a boot drive as well. When I've tried it previously, a good 7200 rpm drive over FW was snappier than the notebook drive. I've got a really deep closet that has a lot of room to breathe, so the plan would be to grab a long FW cable, stick the enclosure at the far end of closet where I couldn't hear it, and boot off of it. In that case the momentus in the mini spins down, the mini is once again a super quiet machine, and I get increased performance. I can use CCC to keep the internal image on the momentus up to date with the one in the enclosure for a quick backup from time to time.
We'll see how well this works. The part I'm most skeptical about right now is whether even a quiet 3.5" drive in an enclosure is going to be quiet enough for me, even if it's 15' away in a closet. I'm sort of a nut.
I do feel it's worth mentioning, however, that for 500 bucks, this machine is really impressing me. I feel like I've spent about that much over the years just on PC silencing stuff alone and have never gotten this close to silence in a machine that was still practical. To have a solution out of the box that does everything I want it do to with plenty of power, and is already a lot quieter than my quietest rig? It makes me really happy. I'm surprised this board isn't completely overrun with people raving about theirs, just for the noise factor, but I suspect if it ran windows that it would be.
Maybe it's also worth saying that I've never been a machead, or OS X nut, or what have you. I've always run dual boot windows and linux. But frankly, OS X is pretty nice. Stuff really does just seem to work, because it's all been designed to work together from the start. Easy to navigate, easy to use, it's good gear. It's not the end all and be all of OS's, none of them are. But I'm happy to say that between the new desktop options on linux (gnome, kde), XP, and os X, there are three totally solid and viable options for your desktop today. OS X just happens to feel like it combines the best parts of linux with XP, and that's starting to have real value for me in terms of time and effort saved.