Elon answered a Tweet about Supercharger abuse with some people leaving their cars parked long after charging was complete. He said they will deal with this situation soon. Evidence of "idle fees" was found in the source code of the Supercharger section of the MyTesla page. Tesla will most likely impose fees after charging is complete to deter this behaviour. The new credit system is going into place Jan 1 so we should have all the details sometime by the end of this month. Expecting a blog post at Tesla.com with all the details soon.