John, there have been a couple of threads on the subject. Most use a subsoiler attachment and fabricate either a "mole" to pull behind it (entire length of pipe/cable is moving through the soil) or a J-tube on the back side where the pipe/cable is fed through (cable doesn't move in the soil).
Here's one recent thread <A target="_blank" HREF=http://www.tractorbynet.com/cgi-bin/compact/showflat.pl?Cat=&Board=projects&Number=154808&page=&view=&sb=&o=&vc=1>Under ground cable</A> /w3tcompact/icons/smile.gif