Well, I mix 5.1 directly out of the PB from Logic and ReWired Reason, into a Dynaudio Air 5.1 rig, I use it mainly for post producing audio to video at the Uni or for mixing music into the 5.1 system then transferring it to DVD-v using AC-3 or DTS encoders, then I can play them on my DVD player at home or in the big teaching rooms at the Uni.
I use an eMagic 2/6 USB interface directly to the active speakers via a Sony DMX R-100 console in the studios, or directly to the analog inputs of an AV combo amp at home.
If I'm feeling especially bad, I'll sling the tracks into one of the ProTools HD systems and have a little fun😀
As melchior says, the DVD player in OSX is stereo only, but other players will direct the AC-3 stream to a digital output (VLC comes to mind) but you will need extra hardware with the digi out on it.