OK, so since this thread was created, I brought up a lot of these questions to the State Park and Rec. They now have a pamphlet out, that describes the rules.
http://parksandrecreation.idaho.gov/assets/content/docs/OHV/IDAHO_OHV_2009.pdf
I'd follow it, print off a copy, and ram it down an officers throat if he jumps you. But, be careful, if your driving down a road, and cross the magic line, you may find yourself going from a county road, to a FS road, and nothing marks the transition.
They are very aware of the confusion. And, I brought up the BS about children's motorcycles/atv plate. They said, if your legal, and riding with a child, and your in control of him, he is riding under you. The pamphlet doesn't cover it. They also said, a child's ATV / Motorcycle doesn't need a plate. But, several people in the room complained that they were forced to buy a plate. Sounds like a total cluster to me. But, the IDPR isn't out to stop families from riding. I would call IDPR and ask them, and ask them where you can get this in writing.
Now, there is another bunch of BS going on, where the forest service is trying to get more forest service roads closed. Watch out for that one.