[Al Skierkiewicz]

Doug,
The limiting factor here is the shunt capacitance of the wire you are using. For 115K and six feet of wire, I doubt there should be a problem. If you are using premade PWM cable, know that there is a high failure rate in the construction of these cables. As others have pointed out, noise from motors and motor wiring will have an effect. This is particularly bad for wiring running in parallel. If that is unavoidable, some teams have reported the addition of a small value cap (.001 mfd or less) at the end nearest the noise source helps. Also you might try using some of those noise cores available from Radio Shack and others. Simply wind a turn or a few through the core to kill noise traveling along the wire.