From James O'Brien of ProHockeyTalk, we have this visual evidence that proves Alex Ovechkin's hit on Brian Campbell came from behind.
Rule 44 in the NHL Rulebook addresses hitting from behind. Ovechkin was given a major penalty and a game misconduct for the hit, which seems to be perfectly in line with what the rules call for.
44.1 Checking from Behind – A check from behind is a check delivered on a player who is not aware of the impending hit, therefore unable to protect or defend himself, and contact is made on the back part of the body. When a player intentionally turns his body to create contact with his back, no penalty shall be assessed.
44.2 Minor Penalty - There is no provision for a minor penalty for checking from behind.
44.3 Major Penalty – Any player or goalkeeper who cross-checks, pushes or charges from behind an opponent who is unable to protect or defend himself, shall be assessed a major penalty. This penalty applies anywhere on the playing surface (see 44.5).
44.4 Match Penalty - The Referee, at his discretion, may assess a match penalty if, in his judgment, the player or goalkeeper attempted to or deliberately injured his opponent by checking from behind.
44.5 Game Misconduct – A game misconduct penalty must be assessed anytime a major penalty is applied for checking from behind.
44.6 Fines and Suspensions - Any player or goalkeeper who incurs a total of two (2) game misconducts under Rule 42 and/or Rule 44, in either the Regular season or Play-offs, shall be suspended automatically for the next game of his team. For each subsequent game misconduct penalty the automatic suspension shall be increased by one game.
If deemed appropriate, supplementary discipline can be applied by the Commissioner at his discretion (refer to Rule 29).
The next logical question is, of course, will Ovechkin be suspended?