A QB who's dumb enough to run with the ball is fair game too. The only exception to that rule is if a QB slides feet first, it's then bad form to nail him, cuz he's already given up. It's kind of a pussy rule but the game gets a lot less interesting with a bunch of injured QBs.
What I ought to point out, though, is that that vid was from the fucking Pro Bowl, the all star game they have after the season. It's generally bad form to lay anyone out in the Pro Bowl, since it's a completely meaningless game. Sean Taylor was an intense motherfucker, though. Dirty fucking player too. Still a shame he died, though. Helluva safety.
Oh, and it's not actually bad form at all to hit a QB, once he still has the ball in his hands. In fact, there's a statistic dedicated specifically to hitting QBs for a loss of yards - sacks.