apple DOES NOT market the mini as an HTPC
whatever thoughts you have about business sense with this are your own, Apple is well known for marketing specific machines for specific purposes and not cannibalizing their own lines, and they see the Apple TV as their HTPC
Right now the HTPC is small business compared to the rest of their lines, and they just aren't invested in it
In time it will be a natural progression to enable audio through the MDP, and who knows, this may be added with Snow Leopard.
Also I would expect that the next iteration of the Apple TV will be 1080p capable and still include a HDMI cable
good times!
(ps I am buying a Mac Mini for use as a HTPC and while a HDMI out or a MDP that transmitted audio would be nice, I'm fine with mdp or mini dvi - hdmi and mini toslink - toslink to my receiver)