Of course, this sort of thing probably goes on all the time; it's just that the Patriots were dumb enough to get caught doing it.
I have to wonder, though, why exactly videotaping the signals that are being sent in from the enemy sideline is illegal. The Patriots could have accomplished the same task by having a third-string QB and a few practice squad scrubs watching the other teams' coaches and take notes, and there's no general expectation of privacy anywhere on the football field except in the huddle. Furthermore, signal-stealing is easily foiled by having a couple guys give off decoy signals; high school and college teams do this all the time.
Perhaps most importantly, if your team's plays can be foiled by having the other team dial up exactly the right counterplay, your team simply isn't all that great. If your defense needs to completely sell out on a blitz in order to get pressure on the quarterback, you should expect to be exploited by draws, screens and quick outlet passes ad infinitum. Great teams can change up their plays after the ball is snapped. If you're a QB and the other team blitzes, hit the hot receiver. If you're on defense, you blitz, and the offensive linemen let you into the pocket and slide off to the side, follow them and don't send everyone after the QB. And so forth.
EDIT: According to this article,
NFL rules state "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'' and that all video or coaching purposes must be shot from locations "enclosed on all sides with a roof overhead."
That was re-emphasized in a memo sent Sept. 6 to NFL head coaches and general managers. In it, Ray Anderson, the league's executive vice president of football operations 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."
I'm not sure if the phrase "locations accesible to club staff members during the game" means locations exclusive to staff; I would tend to think so based on its association with the sidelines, booth, and locker room. Either way it doesn't seem to be mentioned in the actual rule, just in the memo. If that's the case, you could just put some schlub in the stands with a video camera, four pieces of cardboard and a professional lip reader and achieve the same thing legally. That's what you get for implementing a law that has shaky philosophical and practical foundations. Fortunately, the whole issue should be out the window by next season, as the NFL will surely decide to allow wireless communications devices for defenses.