The league policy regarding videotaping was "clarified" prior to the 2006 season with a memo sent by the league office to all the teams, in response to accusations against the Patriots
and other teams.
However, there were some complaints by teams during the 2006 season that the Patriots were still videotaping from the sidelines; Mike Holmgren complained about it during his post-game interview after the Packers lost to the Pats in Green Bay. He called it "annoying" and noted that "it's supposed to be illegal" or similar. So technically they were in violation of the rule at least once during the 2006 season.
In 2007, the contents of the league memo made it into the league's policy manual for game operations. The language was pretty clear, and ignoring the language contained in the policy manual wasn't exactly one of Belichick's brighter moments.
There was never, ever a ban against taping opponent's signals. The Pats would have technically been violating the rule had they been taping the other team's cheerleaders (which apparently happened at least once).
I strongly suggest taking a look at the following link, which gives a lot more detailed information regarding timeline, etc.
http://yourteamcheats.com/what-is-spygate