I came across this claim in the wikipedia entry of EA Sport's Madden NFL Football series:
While appearing on the cover has become an honor akin to appearing on the Wheaties box, much like the Sports Illustrated Cover Jinx, certain players who appeared on Madden video game box art have experienced a decline in performance, usually due to an injury.
Whether or not the Madden Curse exists, EA believes that many fans voted against their favorite player in hopes of avoiding it. The Madden Curse has become so prominent, that it's even gained the attention of the gaming world, creating odds on how likely an injury will occur to the player appearing on the cover.
Also see the (quite long) list of examples there.
The claim is that a player appearing on the cover of Madden NFL is more likely to suffer from injuries and/or a major performance drop in the upcoming season.
Was the number of injuries and bad luck happening to the cover players really unusually high? Or is this just a case of selective perception?