Hopefully it costs you less than six tenths of a cent per hour to run, otherwise you are just breaking even on the power.  I actually have an 8600GT myself - it'd probably cost me about 2 cents per hour to leave it mining, or $3.36 a week.

I suppose it's possible you get your power for free, maybe someone else pays for it.  If someone else pays for your power, you aren't producing anything... could be considered a conquest in petty microtheft, rather than production, as you are simply gaining at the direct expense of someone else, much the same if you were swiping loose change off their dresser or the bottom of their purse.