These music servers are interesting, and may function okay as a transport if the outputs are implemented fairly well. I'd still rather have the full functionality of a PC though. I've been thinking about building a completely silent PC for use a music server and to dabble with PC-based room correction. ..
I started to walk down this path. "Completely silent PC" turns out to be difficult to do. Traditional water cooling is not silent; forget fan cooling as it can never be made quiet enough, and that leaves passive cooling. That's possible (hush technologies has a unit) but in order to keep the heat down, you have to run a slow processor and can't attach modern periph's. And since I wanted to prototype game software designs, slow processors didn't do it for me.
The problem is that you *must* have cooling with modern processors, but the only materials that conduct heat efficiently also conduct noise efficiently. The cheapest solution I found amounted to having the computer in another room, with long cables for keyboard, mouse and video. A bit annoying when you want to change CDs, but of course you only put in CDs to rip them to disk, so it's not that bad really. You will want many gigs of disk space, because wav files are big, and ripping to mp3 just isn't an option unless the source is low quality to begin with...
If people want to turn a HD into a jukebox, consider a Roku. The specifications aren't too bad, and you can leave the computer (or even just a disk server) in another room. I plan to get one for Christmas, I will try to remember to review it.
And if your computer needs are trivial enough, a Roku contains a linux system and with hacks can serve as your internet box...