The Boston Globe had a piece providing some evidence of some of the rules regarding videotaping of signals. Non of which proves the rule was in place prior to 9/6/2006. (Good thing we did not win the superbowl last year!)
http://www.boston.com/sports/football/patriots/reiss_pieces/2007/12/videotaping_rul.html
It reads:
Here are some of the league's rules regarding videotaping procedures:
In the NFL's operations manual, it states that "no video recording devices of any kind are permitted to be in use in the coaches' booth, on the field, or in the locker room during the game." Furthermore, all video shooting locations for coaching purposes "must be enclosed on all sides with a roof overhead."
In a memo to NFL head coaches and general managers on Sept. 6, 2006, NFL executive vice president of football operations Ray Anderson wrote: "Videotaping of any type, including but not limited to taping of an opponent's offensive or defensive signals, is prohibited on the sidelines, in the coaches' booth, in the locker room, or at any other locations accessible to club staff members during the game."
In the league's Constitution & Bylaws, it reads: "Any use by any club at any time, from the start to the finish of any game in which such club is a participant, of any communications or information-gathering equipment, other than Polaroid-type cameras or field telephones, shall be prohibited, including without limitation videotape machines, telephone tapping, or bugging devices, or any other form of electronic devices that might aid a team during the playing of a game."