Here's an old discussion on the subject; http://www.wakeworld.com/MB/Discus/messages/559019/583811.html
Copied from that discussion,
"Jeff The rumor you have heard is true to some degree. At all federal waters you are not allowed to do anything within 20 ft of the back of the boat. I have been stopped multiple times at Powell. I have talked at length with the rangers. The good news is that if you are using a 20 ft rope they are ok with that. Most of the time when they have stopped us we are not using a rope, go figure. But when they ask, we show them the 20ft rope and they send us on our way. I have heard that is wsia is working with the feds to change the rule so you can be standing with in 20 ft. Bottom line get a 20 ft rope and you should have no problem."
"I just got off of the phone with two people at Lake Powell the first was a Brent McGinn, Chief Ranger and the second was his boss, Eric Scott, Downlake District Ranger. I was speaking to Brent first and after a while Eric came in the room so he put him on speakerphone and I spoke to both together. Initially Brent didn't seem to understand what wakesurfing was. He cited the 20' foot rope rule and when I asked him "what happens when they drop the rope?" he didn't think that it was possible. "How do you stay behind the boat? Is the rope attached to the board?" he asked. After thoroughly explaining it to both of them, they both agreed that they wouldn't have any problem with this activity. They both said that they are concerned about the safe activities of the boaters and if they came up and saw this activity, they would deem it safe and move on to the next boat. Carbon Monoxide poisoning is their main issue, I cited the Utah law and they reccommended that I speak with the head guy in Salt Lake. I think it helped out alot when I told them that I have already had correspondence with him. I am going to be sending them a couple of pictures of wakesurfing, they said that they would like to inform their rangers that it is safe, and I invited him (Brent) out on my boat so that he could see how safe it really is. Don't know if he'll take me up on it, but I want to make sure that these guys in charge are not misinformed and apply other appropriate laws (teak and body surfing) to this activity. If I get pulled over and ticketed while I am at Lake Powell, I will be going directly to this guy's office to fight it."