The previous model supported multi-channel audio via the HDMI but did not officially support HD audio.
However I seem to recall that XBMC for Mac had already managed to do this on the previous model Mac mini. For clarification since XBMC for Mac managed to do this it would appear the hardware already could do this but Apple have not provided built-in software support for this.
I could not find a similar statement saying Plex itself supports HD Audio bit-streaming via HDMI.
With regards to the new Mac mini I do not expect there to be any change. That is OS X itself will not support this, Plex will still not support this but I would still expect XBMC for Mac to support this.
PS. The MacBluRayPlayer I feel also does not support this.
The HD3000 graphics did not have the "protected" audio
path required for any HD audio.
When using, i.e. PowerDVD in bootcamp windows, it actually
decoded the HD audio and sent out PCM over HDMI.
On the Mac side, XBMC basically just extracted the non-HD
core from the DTS-HD, etc. and would just stream these bitstreams
over HDMI. It does not decode to multichannel. This is from
actual experience using XBMC for quite some time.
As for the HD4000, that remains to be seen whether the required
protected audio path is present on the new mini.