"The new minis sound better according to mach2music"
Not what was just post a couple of days ago here by Darrell of Mach2 Music. See this thread: http://www.audioasylum.com/forums/pcaudio/messages/9/95090.html
"We prefer the Snow Leopard sound quality overall, but Lion can sound very good with lots of work to the OS."
Their web site may say something different, but they are selling products there.
You are missing the point about the USB bus, there is not an unused port regardless of how many or few external devices are plugged into the Mac mini. Check "About This Mac", then select "More Info" tab, then under Hardware select the "USB" you will see your USB DAC sharing something like the IR receiver, or Bluetooth, etc. This is not the case with the 2010 version as there are at least 2 unshared ports that can be used for the DAC.
Also Gordon Rankin of Wavelength (also in a thread on the Asylum) recommended getting a 2010 Mac mini while they are still available for the reason mentioned above.
My recommendation is to get a 2010 Mac mini while you can, if one needs a Mac mini with thunderbolt that can always be purchased later.
Thunderbolt hard drives will be of little use in a dedicated music server (at this point) application as even Firewire 400 on an external HD is more than fast enough for loading songs into main memory (eg. Memory Play with Pure Music, as well as others).
Noisy fan in a listening room??? No thanks, not for me.
Lion may someday be cleaned up to where it is equal to or better than Snow Lepoard, but that is in the future. Then again maybe never.