Mac Mini as a bluetooth headset?
Solution 1:
You could use http://www.airserverapp.com/ . It basically turns you mac into and AirPlay device for Video, Audio and Pictures. It will not use bluetooth, but your local network which has it's benefits, (range mostly).
Solution 2:
I use the Belkin Bluetooth receiver to stream music to the speakers connected to my older Core Duo mini with the free LineIn by Rogue Amoeba. I've not worked out getting microphone input to the iPhone but am guessing it's might not be possible.